If you hadn't told us that you weren't a native speaker of English, I wouldn't have guessed. You have a good grasp on the language and your sentences are perfectly fine - from a purely technical standpoint. What strikes me as notable is your brevity; you're very quick to get to the point, on every point, and move on immediately once you've made it. Amateur writers tend to make the opposite mistake of sacrificing grammatical correctness and coherent sentence structure for style and fluff, so you're ahead of the game if style is your shortcoming, but it's still going to be something that you're going to have to work on.
I'm going to start by answering your questions, then I'm going to review your work in detail. Pardon the gigantic wall of text that this post is about to become.
Are my sentences awkward?
Yes and no. They are clear, concise, and correct, but they are almost uniform in their pattern, and you tend to end paragraphs on non-conclusive statements. Your last sentences tend to create opportunities for you to continue and elaborate, but you miss those opportunities and leave them behind to move on when a reader might still be thirsty for more detail.
Is the scene boring? Is the scene not suitable for a first scene? Does it make the reader want more? Or is it bleh?
Again, I'm of two minds on this. What you are aiming for is clearly an abrupt introduction without context, and that is asking for a lot of trust from your reader. You can do it, but you need to make sure you keep things interesting long enough for a curious reader to hang on with their questions. If a reader suspects that you are just going to drag them along and not satisfy their need for information in a reasonable amount of time, they will lose interest and possibly abandon your story. If I were to summarize your scene right now, I'd say it's about two kids climbing a hill to get to the top of a rock - not inherently an interesting story by any stretch of the imagination. Now you might be headed somewhere interesting with this as a start, but you'd better make it clear to your reader, even if you don't explicitly explain to them exactly how yet, that this is going to get good and that it will have been worth their time to read until they get there. The excitement of your protagonist, Kira, contrasted with Kurt's reluctance, is apparently your strongest tool for accomplishing that, so I would capitalize on it. The more apparent you make the mix of Kira's eagerness versus Kurt's hesitation, the more readers will wonder why there is such a strong difference of opinion between two people who are close enough to share company in their endeavor. You may know more about your intended plot and be able to offer the reader something more to hold their interest, but for now I'm going to operate under the limitation that what I see is all I get, so bear in mind that my suggestions will be limited by my limited understanding of what's going to happen in your story.
Does it make enough sense?
Yes, from a technical standpoint. We know that two people, one eager and one reluctant, are traversing a distance and scaling a hill or mountain in order to reach an elevated location. It's perfectly comprehensible, we're just left wondering why this is important enough to tell. Readers are a very grumpy sort, and if you don't satisfy their curiosities quickly, you'd better bait the hook or do something to keep them entertained while you draw them toward what you've implicitly promised as being a plot revelation worthy of the time they invested into your story up to this point.
And slightly unrelated question. Regarding past tense of "shine", what's the difference between using "shined" and "shone"?
From
Grammarist:
The verb shine has two main definitions: (1) to emit light, and (2) to cause to gleam by polishing. In its first sense, shine traditionally becomes shone in the past tense and as a past participle. In its second sense, shine is traditionally inflected shined. So, for example, we might say, “The sun shone brightly while I shined my shoes.”
In 21st-century writing, however, the distinction is increasingly fuzzy, and shined is often used where shone would be the traditional inflection. Shone rarely appears in place of shined, though.
I would recommend sticking to this as a rule of thumb. Shone can be used for emitters or reflectors of light, but it gets kind of awkward when you try to stretch it beyond that, and shined can sound kind of juvenile when describing light emitters/reflectors. That can be useful when giving dialogue to childlike or ill-educated characters, so it can work in your favor.
Now, let's get into the meat of it.
(ki: Kira, female character, ku: Kurt, male character)
ki "Hurry!"
I would strongly recommend getting out of this habit. You are writing a story - presumably a novel - and it would behoove you to write your prose fully and properly in the format fitting the medium. Shortcuts like this set you into the mindset of brevity to a fault. I've noticed that amateurs who write in this style tend to slip into the habit of bullet-point presentation, and I believe it is at least in part a consequence of skipping past the usual necessity of incorporating dialogue and corresponding dialogue tags like this. When writing, you need to be as engrossed in your scene as you hope your reader will be, and using shorthand can pull you out of that, turning your writing into a series of annotations rather than entertaining prose. I suggest you treat your novel as a novel from the very beginning. It and your readers are owed that much attention.
I pushed past the tall grass on the way to Red Rock. I glanced back at Kurt. "We'll miss it! Come on!"
The moon-peak was nearing. Already, I could feel the air growing lighter, colder. In the distance, the lake shined.
I've brought up the extreme brevity in your sentence structure, and here is where it first becomes noticeably apparent. That said, I don't want to knock it too much, because within your sentences, your choice of phrasing is good and varied. The only problem is that they read like they started as a series of bullet points which you then simply placed next to each other to form short paragraphs. Fortunately, you've established a few things that will help in turning this around for yourself, and you're not far from where you need to be.
Firstly, you've chosen the first person perspective and the past tense, and you've done a good job in sticking consistently to it. The only issue is that you aren't fully capitalizing on the advantages that this selection of perspective and tense open up to you. Think of what you wouldn't be able to do with a third person perspective or a present tense, and you will immediately realize what kind of things you can do with the choices you've made. Your first person tense allows you to bring the reader directly into the thoughts of the character whose perspective is being used, and the past tense allows that character to speak to events which have already occurred (since the reader is being told about them after the fact) and therefore can allude to other events which, while also remaining in the "past" of the narrative, fall ahead in the "future" of the story that the reader has yet to encounter. Remember, your first person narrator is by definition your storyteller, so the better they are at storytelling - at least at the time of narration, even if not during the course of the story itself - the better they'll be able to help you deliver an entertaining experience to your reader. Use the tools you've chosen, and get inside your narrator's head to share her experiences directly with the reader as she herself perceived them.
For example, why did she glance back at Kurt after pushing past the tall grass? Why did she feel the need to check on him after progressing beyond such a trivial obstacle? Does she doubt that he is either willing or able to keep pace with her? Has he given her adequate reason to doubt his ability or desire to follow? Is she checking on him out of frustration, concern, or a mixture of both of these or even other reasons? Remember, the advantage of the first person narrator is that you can always get inside their head and share their thought patterns and motivations, allowing you to segue from their every action to the next. This will help you fill in the gaps between those bullet points, while also developing your character and demonstrating her personality to the audience.
Finally, you end on "the lake shined," and while you may be worried about whether to use "shined" or "shone," that is not my primary concern with this last sentence. Rather, I would like to point out that and ending like this, even to a paragraph which lies in the middle of your prose, struck me as being insufficiently conclusive to the train of thought. Again, why does the narrator feel the need to point out the shiny nature of the lake at this particular point in her narration? It clearly left enough of an impression on her to remember it well enough to mention it here - shouldn't she have a more vivid recollection of the moment she looked at the lake and saw the light reflected from it? This line is also an opportunity to tell your reader something about the environment - specifically, the time of day. It is not until nearly the end of this scene that you mention moonlight, leaving the reader wondering as to the time of day in which this scene is taking place until you do so. You had a very good opportunity to mention that the lake reflected light from the moon here, but you missed it, leaving your audience relatively in the dark as to whether it was sunlight or moonlight bouncing off the lake's surface. I would recommend reading carefully over the last sentence you use in every paragraph, and if it seems like that sentence might be too short or end the thought a bit too abruptly, start looking into the significance of what was mentioned in the last sentence or what the overall message of the paragraph was meant to be, and try to use that last sentence to tie it all together before stepping off onto your next one.
ki "C'mon!"
I sprinted through the wide field, to the looming Red Rock. My feet kept sinking into the soil.
Once again, you have a paragraph with two not-very-well-connected thoughts, and an abrupt ending. Again, reaching into the mind of your narrator can help greatly in clearing up why she has taken the time to note these seemingly unrelated details in the same span of breath. Or maybe they aren't related, but they are still details she thought worthy of mention, and as such they might be worth exploring just a bit more, so the audience doesn't feel like they're being assaulted with factoids about the story.
The grass gave way to fine gravel, and the ground sloped sharply upward. The flag marking the base of Red Rock stood by the tight path up.
I kicked off my shoes. Above was a ramp of stone, covered in slick moss. I got on my hands and knees and scrambled up.
ku "Be careful!"
ki "No time to be careful!"
Kurt may not be the narrator, but he is still a person with his own thoughts and a means to express himself. And while we may not be able to peer directly into his mind, we can still get the narrator's impression of him through her recollection of his tone, timing, and body language. Whether Kurt is a major character or a minor accessory to this scene, it is important that he be given his due as a living, breathing element who adds life and vibrancy to this part of the story. Because he is not the narrator, you will have fewer opportunities to develop his character, and you will only really be able to do so for him in relation to his interactions with her. You can afford to miss some opportunities to develop your narrator, because there will be an abundance of them. This is not true of non-narrating characters - you will likely need to take every opportunity you can get to develop them for your audience. You don't have the benefit of mind-reading for them.
Past it, slabs of rock were stacked like crude stairs. Moss grew in patches now, and I decided to run. The wind picked up. I leapt three slabs in a row, leaving Kurt behind. I glanced at the horizon. A blurry haze enveloped the roofs of the nearby town, giving everything a soft glow. A feeling of calm enveloped me, and I closed my eyes, letting the wind blow against me.
Moments later, I heard Kurt gasping for breath.
Although this is another example of how your narrator describes things objectively without injecting her own thoughts into the narration, that's already been covered. What I want to get into here is the matter of clarity. I for one do not quite know what is meant by a blurry haze enveloping the rooftops of the nearby town, and while I can be especially dense at times, I suspect that maybe I might not be the only reader who is unsure of what you mean in this passage. Remember, there is more than one way to cause a blurry haze in the real world - when you add a setting with fantasy elements to the mix, it becomes almost impossible for the reader to discern one kind of haze from another. An oil fire and an electrical fire are both forms of conflagration, but the distinction can be important to note. What exactly is the nature of the blurry haze over the town? Is it fog? Is it a distortion in the air from fires? Is it magical or otherwise fantasy-based? If we can't be sure and it's worth mentioning in the prose, we need to have a better idea so that we can have a good image for the scene.
ku "Look."
My eyes snapped open.
ki "Where?"
ku "It's the boundary."
He pointed at the mountains opposite town. Distant fires snaked along the too-far fences, glittering.
Again, we need to know what's going on here, particularly because ordinary fire tends to "glow," not "glitter."
ku "We can see it from here."
ki "Ignore it."
Your protagonist may be encouraging Kurt to ignore it, but now the audience is keenly aware that Kurt may have a legitimate concern. Even if that concern is dropped on Kira's insistence, she can still divulge a little bit about why Kurt feels is it so pertinent to point out the border, and why it should have to be "ignored" in order for the two to progress. Why does the nearby border pose an obstacle? Maybe the reader is not meant to know the full significance of the border, but the reader still needs to be able to understand and empathize with the mood of this scene to a small degree. A clue-in here would be very helpful in drawing the reader further into the scene.
I ran to the next slab and maneuvered up. I waved at him.
You've used the word "up" more than once by now in the context of movement. I would recommend using "upward" in at least some of those situations, as "up" can be used in different contexts, but "upward" is more specific and relates to motion or direction more clearly.
I looked to my side and saw Kurt's flushed face nearing the step. I smiled at him, and he looked away with furrowed brows.
"Furrowed brows" is one of those phrases that is good in its own right, but has unfortunately seen a great deal of overuse from amateur writers, and as a consequence has grown tiresome to readers. I would be very, very careful about applying this phrase to any part of my writing, and would use it only if I were absolutely certain that it was the best situation to use it in. You don't want your prose to come off as too purple in the wrong places, and commonly overused phrases like this can create that impression with just one appearance in your writing. This can be a pretty unfortunate impression to create, especially in writing with as much brevity as yours already normally has.
ki "It's a good idea, I promise."
I sat myself up. The flat top of Red Rock lived up to its name--unlike the slabs, it was devoid of moss. Under the moonlight, it was the color of terracotta pots. Kurt was still below, catching his breath. I hoisted him up.
I walked over to the cliff's edge. Beyond it was a steep drop to the lake below.
ki "We'll jump on three, okay?"
ku "Okay, on three."
ki "Come here."
ku "You go first."
I marched up and grabbed him by the elbow.
ki "I'll count, and we'll jump together."
He nodded at me, and I let go.
ki "Okay."
There's a clear conflict here. Again, this is a great opportunity to expand and explore into the mind and perception of your narrator, and of Kurt's behavior. If you breeze right past this stuff, the reader will think it must have been unimportant. And if they think that, they will wonder why you bothered to mention it at all. Stories are mostly a collage of highlights, and readers expect everything they're told to have purpose. If it doesn't seem to, they'll feel annoyed at the thought that they just paid attention only to receive extraneous information that they didn't need. That leads to boredom, which leads to skimming, or losing interest entirely. Don't forget that you are constantly justifying to your reader why they should continue expending the effort in reading your story.
The wind stopped.
ki "It's time."
ku "Already?"
I nodded.
ki "Let's do this."
ki "One..."
I felt Kurt tense up.
ki "Two..."
I looked straight ahead, at the night sky.
ki "Three!"
I will say that this last line of dialogue is a suitably abrupt way to end a scene before leading into the next one. If there's any place for an abrupt ending, it's at the conclusion of a scene. You did a good job closing this one off, it just needed a more vivid buildup so that the reader could be invested enough by this point to really care about the moment these two characters make their jump.
You seem to have a fair idea of what you're going for, and I trust that there's a deep story waiting for the reader beyond this scene. What you need to develop is your sense of the reader's curiosity, interest, and need for entertainment and information. This is more of an art than a science, so you'll have to feel your way around it and place yourself into the position of a reader as you reread what you've written. That is why, again, I strongly recommend getting and staying in the habit of writing your prose in the novel format, and not in shorthand. Get used to presenting your words as though they are all the reader will have to go on, even if you are planning for a Visual Novel. The additional effort it will take - and really, it's not much once you get used to it - will pay off well for your audience and for yourself.
