I'm going to rip into this one a bit. Nothing personal.
Your story seems to have taken a long time to get to the title character which is an odd choice. It's not hard to include a new character in your story. You just have to have them actually MEET the main character. Characters can meet anywhere, especially if they're going to the same place at the same time.
Also, your take on Hisao seems extremely melodramatic.
Throughout all the arcs but (I believe) Rin's and Emi's, we see Hisao neglecting his pills and his health. This is a lamentably poor decision and seems, to us, pretty damn stupid. After all, isn't he going to die if he fails to attend to himself?
Death is ever-present for him, and that's a very difficult concept for a teenager to suddenly find himself with. There are three ways to deal with something of that magnitude: accept it, try to escape it, or let it mold you. For evidence of the first you can look to the Hisao of Rin's arc, at least in part. For the last check out Rikabro's Rika arc, which I've linked in my OP.
Hisao is unable to cope with this sudden change so he tries to run away from it. Taking pills to him is an admission of his own mortality and his own changed state: I am going to die unless I take these pills. Running is like saying because I have arrhythmia, I have no choice but to exercise. This outlook changes with RIn when she forces him to look at himself honestly.
Frankly, that's garbage.
First, neglecting his health isn't consistent with the character as you've written him. If anything, he's overly preoccupied with his health. He might not want to exercise, but he realizes that he's going to DIE if he doesn't do anything, and swallowing a few pills is hardly a strenuous or time consuming activity.
As for neglecting his health, he does at times, but the only times he forgets his pills are when his mind is really focused on something else. He doesn't think “Oh, my pills. Well, they aren't important.” he just forgets them altogether because he's worry about something else (and even this is very rare.)
As for using the Rika story as evidence, it's both not canon and it shows Hisao in an incredibly self destructive relationship that even he acknowledges.
I did think it was an interesting choice to have Hisao obsessed and mystified by this mysterious girl (even if it did seem forced and out of character for him) only to have her being direct and down to earth when she finally got lines. I groaned at the fact that she was American though. I don't think I've ever seen the American transfer student done well.
Why did my parents give me a towel if I didn’t have a bathing suit?
psst You're supposed to use them after you take a bath, Hisao. You have been taking baths, haven't you?