Adding to the comments on the letters: are they delivered by post? Deposited in a locker, bag, or similar space? I don't think the premise is outlandish or anachronistic simply because of longhand letters, but
Each letter made him feel like he was in their shoes and all the emotions within the letter, he felt all of them. It’s now up to MC to discover who the sender is and possibly even express his love.
I'm finding it hard to believe that someone so empathetic with the sender wouldn't be able to guess their identity among his friends/classmates (this is assuming that it's one of the girls).
As for your questions:
How do you feel about the potential love interests? Who was your favorite and why?
I'm iffy with a few of them:
Sanae: A reclusive writer who's somehow amazing despite never showing her work to anyone? It's not impossible, but the first writer who comes to mind is J.D. Salinger, whose
The Catcher in the Rye I rather detest. A poor first impression, to say the least.
Most writers improve through feedback from mentors, peers, or their readership, and rarely in their youth.
Hanako: Shyness seems to be the only trait you've allowed her. If that's how it is, this one's not interesting--but I'm assuming there's more than meets the eye (which perhaps you're withholding for reasons of dramatic tension?).
As for my favorite, that would be Ayaka. Intelligence stemming from studiousness and generally pleasant company; good conversation is always a plus. The lack of any singular defining trait makes her seem more substantial, if that makes sense.
"A good flirting session" has me a bit confused, but I'll have to see what that means, I guess.
Are you interested in playing this game, why and why not?
Would give it a shot, though as my comments on the characters show, I'd be selective about the routes.
Additional comments/questions:
I am interested in this "investigative system" you so innocuously mention at the end of your post, but skeptical about the Filipino translations.
Looking forward to seeing updates on this one.