Head!canon and True!canon, a chaotic mix in one entry
Shiro is a bastard, no, really.
When he knows what is going, he's friendly, seemingly careless and, maybe, somewhat of a dunce?
He's always, always sloppy. Except when he isn't-- see, fighting, drawing.
When he doesn't know what is going on, he's more honest, moody, delighting in driving other people up the wall.
On the same note, he hates being bothered by others.
Shiro makes it his business to know what is going on. As Koujiro states, he has his ways.
Then again, he never expected to end up anywhere like the Thor. He needs to get used to that, but currently he finds himself out of his elements.
Still, don't expect him to be completely clueless about things. He eavesdrops, analyses and...uh, stalks.
He fails at all things technical/electrical --but is too arrogant to give up.
Despite being paranoid, he is and stays a lazy sod.
De facto that means that Shiro does what he sees as necessary- getting to know his surroundings, for instance- but still lazes around afterwards.
He doesn't butter up to people.
When he fights, he doesn't care about getting wounded-- which is his weak spot.
He doesn't take good care of himself anywhere else, either.
He can and will look after children and be good at it.
Children charm him.
Adults...not so much.
Puppies might actually be something of a trigger for him. Would be mine too, if my other dozen brothers or so had been trying to eat the one I'd taken in.
He spend his childhood looking like a mummy, bruised and bleeding and being trained to be a perfect killer along all his brothers.
The only reason why this manga isn't a sob!story is because all these flashbacks are shown very shortly and then disregarded.
And because everyone is crazy quirky.
One of Shiro's own major quirks-- don't ever tell him to take up a role. He'll overdo it like woa.
Canonwise he was only ever interested in two people, his brother and Koujiro.
Even after he run away and tried to live a life without killing, he was planning to murder his brother.
It isn't that he hates his brother, but Anri is still a psycho.
Also, he'd like to set him free.
Having his home planet blown up kind of tore down his walls. He will find it easier to be interested in people, but often it will be an act.
Shiro crushes, despite all that, terribly easily on people. Terribly fast, too.
He never pursues anything and doesn't mind if he'll never see that person again.
Sex, in his eyes, is about power.
He visits prostitutes, and is very nonchalant about it.
He's also rather friendly to them.
To most people who'd had it rough, actually.
He does do regret.
His brothers, of course, were not his blood-relatives. Just other children who were kidnapped/picked up and trained.
He has this fixation with denial...if he were to like someone, he wouldn't see it.
Ignoring all snide comments, arguing and curses, Shiro likes people who are headstrong, have morals and fight with their all.
Shiro uses/mixes poison. Once, when Anri asked him to draw something for him, he drew said something directly on his skin instead of canvas and later told him, gleefully, that, ah, yes, that was poisonous paint.
Gets called a sadist. And good-looking. Ahuuh.
He can play the shamisen.
Has the habit of always carrying small and pointy things around with him.
His home is stocked with them, too. Hidden who-knows-where.
You better don't trust the umbrella he carries.
When he is shaken for some reason, he can suddenly act all concerned and open. And he starts to yell, which he usually...doesn't do.
DID I YET SAY THAT THE WHOLE STORY REVOLVES AROUND HIM HATING UNDERWEAR?
So, yeah, expect him to go commando, always.
All in all, Shiro is just very human. More open around people he knows. He blushes then, argues and never gets anyone to listen to him.