Some of it's head canon based on canon things. Other parts are random bits of canon, but here's just little blurbs or explanations for some of the things Seto does/acts/etc.
HAVE I PLAYED THIS GAME TOO MUCH YET?
1) Seto is left handed.
2) Obsessed (borderline stalking) with the silver haired girl, Ren. He deviates from his mission of going to the red tower to find survivors, just to follow after her. Out of all the graffiti in his world, he's able to discern what doodles/drawings belong to her.
3) He will defend her HONOR, even over something as simple as a drawing. "It's not scary, it's cute."
4) If finding Ren is involved, he will do nearly anything.
5) The quickest way to get him to flip his shit is to steal the locket around his neck. It's extremely important and he will chase someone all over the world for it. It would take a long time before he would show anyone what's inside, even if asked. The only person to see the inside is Crow because Crow is a jerkass and stole it.
6) Due to canon events, Seto is under the assumption that to become best friends you have to kiss. Unless someone tells him otherwise, that's the only way to get into Best Friend Section. He's not one to go around kissing people, that kind of affection freaks him out unless it's coming from Crow.
7) He will not talk about dead friends. He might mention them on occasion like (old friend, once had a friend) but no names will be mentioned. If they're dead, they're dead. He's very aware of that and sees no point in bringing them up.
8)His mourning period for any death, even if he knows the person well, is very short. We're talking about a couple hours/a day, but every dead person/person that disappears is fondly remembered. It might come off as cold, but he's a firm believer of 'the living must continue to live'. He does hold a high respect for the dead.
9) He has a low reading comprehension. There are foods, kanji, words that he just can't read or doesn't know. He can read picture books and simple words like (Risk, Danger, Exit, Die.)
10) He knows what happened to the people on his planet. Seto doesn't mind answering questions about his world, but may avoid the 'how it happened' aspect. Mostly because of Shin's involvement. If a castmate were to ask, he'd tell them. Same goes for anyone he gets close to. (Orange/Red heart) Seto's aware that his 'Grandfather' played a hand in their destruction as hinted at by the books in the archive room and Sai.
11) He can see ghosts (aww yeahhh fun times)
12) He lived outside of Tokyo. (There were Tokyo newspapers on the walls, Azabudai was one of the first places he visited, and he's headed towards the Tokyo Tower for a majority of the game.)
13) In the game, he made a promise to a girl on the 2-way radio. The memory of not being able to keep it haunts him even on the Thor. While he's on the look out for Ren, he's on the lookout for that girl too. Seto does not like breaking promises.
14) He does not like dogs. Prior to the Thor, all dogs were bloodthirsty beasts. Real dogs and Malicious Thought Dogs. Recently, he's come to terms with the fact that dogs can be nice, when they're puppies at least, but he will still refuse to go near one if it all possible. Jellyfish are assholes. Birds are jerks. Fire throwing screaming spider things are not cool.
15) While he would recognize the language he speaks if someone were to talk in Japanese or write it out, he doesn't know the actual name of the language/country he's from.
16) Seto has no recollection of any blood relatives. He never mentions them in game or gives them any thought. His 'Grandfather' was, in reality, just an old man that happened upon Seto and gave little orphan boy somewhere to live, possibly out of guilt.
17) Friends are serious business. He will talk/hang out with everyone, but it has to be outright stated they're friends.
18) About Crow: Despite how Seto may go on and on and on and on about the silver-haired girl, Crow is extremely important to him. Seto does not ''love''-love Crow despite how they act. If asked, Seto would say that he loves Crow because he doesn't understand there is more than one way to love another person. When they kiss it's just a way to maintain their friendship as biffles/comfort, not romantic. Seto will leave Crow to do his own thing, but frets endlessly if he doesn't hear from him for a few days.
19) He has vegetarian diet, considering he didn't hunt and the pre-packaged meat on his world was not safe after like 10-15 years of being left alone. He'll eat leftover food on tables, scavenge for food on planets and you know that half eaten bag of chips that was on the floor in the hallway earlier? He ate that too.
20) If your character introduces themselves with a first name and last name, he will assume that is their entire name. He has learned what last names are from Cruz, but is unable to tell the difference. On that same note, formalities are lost on him. -San, -Kun, -Chan, etc confuse him.
21) Seto made his own map of the ship, the floors, and everything in it. Rooms of his friends are marked off with smiley faces. Yes, he knows where you all live. Scary places are marked off as scary.
I'll periodically add to this.