--> Haruki's forms of address tend to vary. For women, he uses LastName-chan, for men, he uses FirstName-kun, showing somewhat of a separate view on how much he likes them. He will change what he calls someone if they demand it, but he can sometimes slip back into his habits anyway.
--> He likes to read, write, and make things out of clay, and is pretty good at making little statues. He also likes to cook, but he's so awful at it that he feels guilty about doing it, since it wastes food.
--> He doesn't think very much of himself. He's not as self-degrading as Komaeda, but he's aware most people think oddly of him so he doesn't believe he's worth much to others. It's not that he believes they'd not be sad over his deaths or miss him -- they're his friends, so that much is a given. It's more that he doesn't think they'd want him as a major part in their life, so things like lifelong friends or dating them just seems unlikely at best to him, impossible at worst. As such, he's not bothered by things like "well they won't be your friends forever" or "well they won't date you", because he feels that's a given.
--> He does his best to come off as cheerful and friendly as he can, mostly because he's trying to bear the burden of misfortune without passing it on to anyone else. He's also very friendly to most people, even outright. He tends to talk in a calm voice, though he can shout when he gets excited.
--> However, he can get nasty and rude with things or people he doesn't like. He can sometimes hide his fear for a little bit if it's something he's used to, but if he's caught off guard, it's impossible to stay composed.
--> He's snarky in general, though, and can sometimes poke fun at someone if he feels like it.
--> Strangely, his voice and expressions can seem innocent and almost childish at times, despite the maturity behind them. Most people assume it's because of how young dementia hit him, though it's impossible to say exactly why.
--> Haruki can hide things when he gets scared at times, but only if it's minor enough and he's been forewarned. The first time he's scared, or if it's too much for him, he'll snap and collapse or freak out, often bursting into tears. He can also hide it if he's focused intently on something, but once the focus goes away, so too does his composure (leading to odd scenes of him carefully explaining how to get through some awful situation... and then bursting into tears once the situation is over).
--> Like his father, he can be scarily perceptive about some things, which is amusing to some of his classmates considering how one-track Haruki can be at times or how focused he gets into certain things.
--> He understands why people might hate him, but he considers his classmate friends even if he thinks they want nothing to do with him, and will do what he can for them because of it. For nonclassmates, he accepts that people might be using or mocking him, but he likes to still at least try to think of them as friends even while accepting that. For Haruki, it's too lonely otherwise.
--> He tends to be very physically affectionate to those he cares about.
--> Haruki has always been obsessed with one of his fathers (Komaeda Nagito). While this obsession has always been platonic, it came from the fact that Nagito was deathly ill and essentially living on borrowed time, able to die at any point, and the fact that he was always somewhat clingy and dependent on him. It was odd, if not entirely unhealthy at the start, but an effect of his dementia was to make him even more paranoid about it. Even with treatment, the paranoia hasn't eased.
--> He takes so many pictures because it allows him to share memories or views with his father, and he finds that important. There is another reason, too: Haruki sees pictures as physical proof of something happening, essentially acting as a "backup" for a memory. If something makes you forget, the picture holds the proof of the happy time, of the fact that it was real. It makes it so that time, and whoever is in them, can't be forgotten. With both him and his father suffering dementia, it's a huge comfort for him.