Ikkaku is a thug. He's...actually pretty amoral when you get right down to it--he drinks and gambles and is obnoxious and also a total hedonist and will, in fact, bust someone's shit up if they look at him wrong, he's kinda completely retarded, and moral codes as they apply to the world at large don't exactly apply to him. He yells and belittles and bullies and is, in fact, completely used to getting his way. Also, no manners. At all.
At the same time, he's a nice guy with a very strong sense of honor. Ikkaku is a man who repays his debts in full--if you spare his life out of the goodness of your heart then he'll get pissed off, sure, because people aren't exactly supposed to spare him, but in return he'll likely take the very next opportunity he sees to cut your ass some slack.
Ikkaku loves his life. Fact. He loves his job, he loves the people he works with--his division is his family. And since he's unafraid to just take what he wants, he...well, he gets what he wants. Ikkaku is happy. In accordance with this, if he died at any given moment then he'd do it without any regrets whatsoever. Yes, he does have a death wish, and the fact that there's not a lot along the lines of "things I will regret" or "things I have left to wait for" exactly holding him back only amplifies it. He wouldn't be able to stand dying quietly, so he's looking for his last fight to go out with a bang.
Ikkaku doesn't consider a whole lot of people outside his division his friends--he's detached becaus he's kind of nuts and as a god of death he's obligated to simply pass through people's lives without a hitch or consequence. He doesn't think he has huge impacts on anyone's lives unless he cripples them for life or something, and he's a-okay with that because he doesn't give a crap. He's paid to protect humanity, and he digs humanity, but they're on a separate plane of existence. Even if he hangs around them every day now, there's a filter in his head, separating "me" from "them," and this is why he treats a lot of people in pretty much the same way while still claiming to see them as individuals.
People Ikkaku actively considers stronger than he is aren't allowed to die before him. This includes but is not limited to: Kenpachi, Yachiru, and Ichigo.
Ikkaku is actually pretty bright! Problematically enough, though, especially when it comes to fighting Ikkaku is looking for the FUN WAY OUT rather than the smart or logical way. When it comes to fighting, Ikkaku will never, ever go all out unless it's really, truly serious business like the Arrancar fight where it's a specially ordained mission and failure brings the universe one step closer to the apocalypse. Other than that he's kind of like one of those cats who finds a mouse and goes all GLEE about it and likely kills it entirely accidentally. Whoops.
If an ordinary person was marooned on a deserted island they'd probably get a fire going and think about sustenance and all that Tom Hanks crap; if Ikkaku was marooned on a deserted island he would swim and likely not get tired of swimming for a very long time. Or blow up the island. Or hijack a shark.
Ikkaku likes himself. No, this is a lie. Ikkaku TOTALLY LOVES HIMSELF. Ikkaku thinks he's just about the coolest thing to ever happen to anything ever, and he just can't help being FANTASTICALLY SEXY, and you're all just LUCKY TO HAVE HIM HERE.
RUKIA
Rukia speaks very formally. There are two reasons for this. Early in the manga when Rukia first enters Ichigo's school, she speaks like a total freak--rude and old-fashioned. Which basically means Japanese isn't her first language and she did, in fact, learn in a day. Soul Society likely has its own super-special language where everyone understands everybody, but--yeah, she doesn't have much of a grasp on informalities and contractions and things because she had to learn quickly. This does carry over into camp, where--well, I dunno what language everyone is speaking, but it certainly isn't one she knew before, and the language she does know is likely one not a lot of people know.
The other reason being her upbringing in the Kuchiki house, where Byakuya is Byakuya and everything is formal. Part of the noble house thing. In academy if she spoke in a disjointed fashion it'd have been because she never had any formal education; however, Byakuya speaks practically in haiku, and while Rukia is no poet she did adapt that old-fashioned style and those mannerisms in order to be suited for the house.
Rukia has low self-worth, yeah, but if there's anything she has confidence in it's her skill. Skill in fighting, skill in demon arts--she trains diligently and does feel like she has something to show for it. Skill in art too, but WE DON'T TALK ABOUT THE BUNNIES
...she doesn't consider herself strong, exactly, but rather capable. And while she claims she doesn't do frivolous things, she isn't adverse to showing off if asked nicely ♥
Rukia is a little strange about formalities. To their faces for the want of at least seeming polite, she will refer to people formally--honorifics, last name, the whole shebang. When she's not around them, though, she'll refer to them with whatever name she feels like referring to them with. This being because she makes the distinction between humans and nonhumans, like Ikkaku except a little more prominently, and she adheres to human customs--honorifics being one of these customs, in a way--while she's around humans and proceeds to operate by her own rules when she doesn't feel the need to make the distinction anymore. As in, she'll call Rabi "Rabi-san" to his face, but if someone else talked to her about him she'd refer to him as just Rabi. She also uses honorifics to mock people--a case of this in camp being
When I think of Rukia not using honorifics at all, to a person's face or not, the immediate examples that come to mind are Ichigo, Renji, Orihime, and Hanatarou. This being because she trusts them on some distinct level and she likes to believe they trust her.
Rukia is very quick to using honorifics with people she adores and respects right off the bat, and the magnitude of that adoration sort of expresses herself in that form. Shiba Kaien was immediately Kaien-dono; Gai was immediately Gai-sensei. When she respects someone, she respects someone and will defend their honor if insulted and...yeah. She has her idols.
Rukia loves Byakuya. Fact. Rukia will defend Byakuya's honor from anything that might impede on it. Rukia misses her brother but is hesitant to even consider the idea he might miss her. She admires him more than anyone else. She writes detailed reports of her time at camp addressed to her brother. Byakuya is her family and, post-SS arc, they really are starting to act like it.
Rukia loves Renji. Fact. It's...actually pretty complicated on this end, but it boils down to them being a unit--after fifty years of separation he's back in her life where he belongs. Before Byakuya, Renji was her family, and they've been through a lot when they were apart--but they've been and they're going through a lot while they're together, and they fit.
Rukia...does love Ichigo. Very strongly. F-fact. It's...not a romantic love, no. And he isn't her Kaien and she isn't his mother, but--he's her hope, he saved her, she believes in him and isn't afraid to rely on him.
At the same time she doesn't need him and he doesn't need her, and she can function in Soul Society where she belongs knowing he's doing well for himself where he belongs.