who is more likely to get injured doing small tasks – Emmet, most likely. He can get a bit overenthusiastic, and when he’s on a roll he can not realize if he’s hurt himself in that enthusiasm until Ingo or Concordia notices and makes him sit down so they can get out the first aid kit.
who worries more about the other – This is a hard one since all of them have their own worries over each other in different ways? Concordia worries that she can’t always keep up with or be there for her partners since they’re so different (both in personality and species), the brothers worry about each other like they’ve always done, but also worry about Concordia, but then in different ways. Emmet worries about providing for her and making her happy even though she’s a human and they’re not. Ingo worries that being with her will throw off the dynamic he and Emmet have had for centuries… at the same time, they’re all pretty good at communicating, so these worries usually don’t fester.
who is more afraid about the other leaving them – I’m going to answer this with a curveball and say it’s their son Janus who worries about his parents breaking up. As in, while Concordia loves her partners, she knows that their relationship might not last and is happy to enjoy it as it happens, while the brothers know full well that their relationship with Concordia is going to be transient in the long run (as these things always are for them and humans). But Janus has only known his parents being together, so he dreads the day when that might no longer be the case and as a result can be clingy about wanting all four of them to spend time together.
who is more likely to leave (for any reason) – Concordia. Emmet and Ingo are ride or die for each other and always have been, while she’s the newcomer to a long-established dynamic. Half the reason she even moved in with them rather than keeping things casual was because of Janus, so if things ever start going south, she’s the one who’d step back without much fuss, taking things with grace as she always does. (This doesn’t mean the twins won’t want to try to convince her to stay though! They definitely have both come to care about her a lot even if such feelings were maybe unexpected.)
who is more likely to drunkenly confess – None of them? I can see Emmet and Concordia both being affectionate drunks though (Emmet more clingy and Concordia just wanting to cuddle and fall asleep). Ingo has to carry them both home haha. 
who is more likely to push the other away (for any reason) – Ingo can be standoffish when he’s in one of his moods. Especially earlier on with Concordia, who he had reservations about. She and Emmet were originally just seeing each other, but Ingo is a package deal with Emmet so he got involved and let’s just say he wasn’t sure getting so close with a human like her was a good idea. Would just invite trouble.
who picks fights more often – None of them really fight much? But again, Ingo I suppose. Can be surly sometimes, but his fights are almost always with Emmet and are the usual arguments they’ve always had that get resolved quickly. Concordia is scary whenever she gets involved with a fight so the twins have both learned not to test her patience. Or both will be sleeping on the couch. 
who usually apologizes first – They’re a weird trio where they don’t really apologize with words but with actions? Make it up after a fight by doing something nice for the other(s). So it’s hard to tell who’s usually the first since it’s often mutual.
who is more likely to withhold their feelings for the other – Ingo and Concordia for sure. The slowburn branch of the relationship, while Emmet was very quick to grow infatuated with Concordia. She on the other hand was wary of growing attached to anyone after her upbringing, especially people so… different as the twins, while Ingo had similar reluctance. So obvs Emmet just decided to play matchmaker between the two so all three could be happy! Somehow it worked. He’s just talented like that. 
who is more likely to lash out at the other – None of them, really. 
who gets more jealous – Ingo, sometimes, but he refuses to acknowledge these feelings. It can be hard to share when you’ve been with someone exclusively for so long.
who is more likely to support the other in a relationship with someone else “as long as they’re happy”: – The twins for Concordia. Both of them know they’re probably not the most ideal boyfriends for someone more normal like Concordia, so if she found a nice human person to settle down with… that’s fine! Even better! (Just as long as they can still see Janus though, of course. That boy is spoiled and doted on by all his parents.)