Random, Non-background related headcanon:
He hates using statistical probability in cases - as far as he's concerned, it's asking to find yourself tripping over that 2% that doesn't match the "98% of X crimes have Y background somewhere" and make an ass of yourself.
He's morally altruistic and ethically pragmatic - Occasional moments of Not Being A Saint aside, he's mostly committed to doing the right thing by people, but a mix of factors have him a little jaded about the Letter of the Law, and if something like Normal Social Standards, Laws, and Cultural Sense of Right And Wrong clashes with his assessment of the situation, he will ignore, skirt, or outright break the "law". This is kind of canon, given that he basically tampers with evidence on the Kid case, and... I need to actually double-check, I'd been under the impression that yes, he did own the gun he had in Sunset Manor, which is in Japan, where civilians aren't legally allowed to carry firearms. (Part of why he keeps it on him in Japan anymore is paranoia - he knows about the Black Org, and he'd rather not rely on the smoke grenades against them... and yes, he DOES carry it concealed on Kid heists.)
Given the chance, he's slightly fussy about some things; he'll prefer good loose-leaf tea over teabags any day, or maccha for green tea. In spite of this, thanks mostly to how much of his childhood was spent hanging around police stations, while he'll eat pretty well given the chance, he can be a bit of a garbage disposal and fairly tolerant of "whatever is available". This is not helped by the fact that he cannot cook; he's used to that being taken care of, and is prone to making stupid mistakes that'd be common sense to someone used to it - turning the heat up too high, not properly draining tofu before trying to fry it in oil, etc.
On more intense short-term cases, he's just slightly obsessively focused - get him on a serial killer chase, and as was demonstrated during B's spree, he basically only sleeps as much as necessary, will forget to eat/only remember to eat as 'something to maintain ability to function', and isn't going to be distracted by very much outside of it; he basically will crash and get back to his normal OCD self-maintenance afterwards. (Other than that, he usually actually takes good care of himself.)
He likes toffee, caramel, and some fairly strong-tasting foods, including sour candies. He also likes seafood, which is something I'm sure Kaito must love.
He has an accent in just about any language. His English is the best, although it's pretty clearly London-British (there is some MUN fail on this sometimes) - there is a slight Japanese accent, but it's faint enough that it mostly comes across as "his speech sounds a little odd" without enough for most non-linguists to pin down what's off... granted the rounded L/R's and occasionally dropping R's on ends of words might tip some off. His Japanese has a pretty noticeable British accent, which is actually a plot point in canon in the Koushien. His French is weaker and gets hit by both, as does his German, and what fragmented Welsh and Russian he manages. (He can follow a little and speak enough to navigate somewhere, but he can't really carry on conversations well in either of those languages.) He can swear in more languages than he actually speaks - side effect of his "job" and what he's gotten exposed to.
Thanks to his reputation, he's pretty used to dealing with reporters, and fangirls, even before he got tangled up in the Kid case ... which basically compounded esp. the latter problem exponentially. He also regularly gets the short straw on "Keep track of Kid's fan club to see if he's doing anything that angle/if any of them are getting any stupid ideas" duty from the Task Force. He tried to pull a few tricks to make himself less easily recognizable and play "alias name go directly hang around larger groups of the fan club" a few times ... it kinda failed. He got recognized. He considers those incidents mildly traumatic, in a noodle-incident kind of way.
He deals with relationships funny. Basically, as much as he's not that close with many people, he'll get very strong attachments to a small margin of people, and it can easily tip over into flirting depending on the situation and the person in question. He's still mostly unsure what to do with the idea/possibility of "serious relationship", and his generally chaotic personal life hasn't helped any. (Also yes, he is kinda bi, although he's more prone to flirting with girls - generally easier, plus I get the impression he's used to ending up competing/butting heads with guys in his age range, which tends to make that more situational and case-by-case. No, Kaito wasn't entirely wrong in making stalker-with-a-crush jokes. He's pretty used to spinning his wheels and not expecting to get anywhere. Also yes, he does have a pretty high tolerance for odd behavior.)
Comedic note on that point: yes, the occasional slightly predatory moments at Certain People Of Interest ARE things where he HAS been halfway flirting for some of it. (No, he wasn't actually flirting with Light, although he was perfectly happy to make it sound like it.) He doesn't do that very often, but is perfectly self-aware enough to realize that it's a demented sort of expressing interest. (The more confusing the signals/frustrated he is, the more likely he is to get snarky/pointy.) HOWEVER: Thanks to being mostly raised by Baaya if anyone, who is kind of MUCH older generation, he's kinda had a chivalry complex drilled into him!
Some of his random flattery and being overly chivalrous to girls isn't really trying to flirt.
This has not helped his problem with fangirls and getting half the girls in his high school class deciding to try and get his attention.
If you are female, you will have to try a bit harder to get half the snark guys will get. It is instinctive and is backed by Fear of Baaya Who Would Use His Entrails For Garters If He Mistreated A Girl.