It's been a while since I last read anything post-apocalyptic, and it's not exactly my favorite genre. Still, I'm willing to give this one a go

It'll be interesting to see how Rish and Ryter (icwutudidthar) communicate with each, if they do at all. As for comments on what you've got thus far, I'll number them.
1) The basic premise itself isn't all too unique; humanity goes to war, nearly nukes itself to death, animals and plants possibly mutate, and only a few survivors are left. It's all been done before, especially the part with the guy meeting the girl. However, the thing that
does stand out is the fact the guy can talk but cannot read/write, while the girl is the exact opposite. This sounds interesting to me since it'll pose problems and lots of misunderstandings when they first meet, and I really want to know how they'll get over this communication barrier.
2) I really like those character designs, though I think Rish could use a couple scars :3 The only two things that's offsetting me a little bit is that their pants aren't as tattered as their clothes and the hands look kinda tiny (but maybe the hands part is just me :/). For a story like this, your current style (just shading in everything) or muted or faint coloring would suit it just perfectly. I can see the difference between the sprites and the background just fine. If you do decide to color it, dab on a little skin tone and some subtle color on the sprites and stick to plain earthy tones for the backgrounds. No
bright, fluorescent colors that stand
out a lot, or else it'll totally break the mood of the whole thing. Also, the buildings and whatever remaining structures in the background should be more dilapidated; post-apocalyptic means that no one is going to have time to make sure everything is nice and pretty.
3)If I were alone at the end of the world, I'd probably get really depressed D: If there are only a handful of humans left (I'm talking 70 or less people out of a world that is 7 billion before massive genocide or what have you), there's little chance that humanity can rebuild itself back to pre-war status. Procreation would be slow, death rates would be high thanks to disease/starvation/whatever, and even if humans do survive long enough to get a new generation in, eventually everyone will be related to each other and the chances of genetic mutation increases due to some technical-term-I-forget-the-name-of-right-now. Yeah, just thinking about it is enough to make me depressed :C
4)It would be too much of a coincidence if the last two remaining people on earth happened to bump into each other, and it just so happened that one was a girl and the other a boy :/ Maybe you can mention other groups of people (like Rish has seen a couple people band together and form small camps, but he didn't like the way they did things), but this is up to you.
5)Is Ryter mute or can she speak in a different language? Just curious to know.
So I poked and prodded at anything I could think of to pick at, so hopefully it isn't TL;DR status. Good luck, and I'll be
watching you supporting you c: