Well, his bias as a child followed him long into adulthood that he never stopped to contemplate on even when Korean Girl was desperately fishing for their love, and he does have a dangerously disgusting possessiveness of her (“You NEED to rely on me because I’m all you have” with that kind of weird self-satisfied smile? And getting jealous at the thought of her having other support systems when he hasn’t been one in years? Eughhhh…)
You are right that he is (so far) the only remorseful one around her which minutely softens our view of him (the dad is being very complacent with most of Kanna’s demands but I hesitate to pop him in the remorseful category right now). But I’m with Kanna that it still feels a little self-serving. Especially since he was denying it for so long and being like “I’m not THAT bad” and that he’s working to protect the blonde kid from prejudice but never once stopped to realise that Kanna was going through the same thing when younger up until the present.
I do look forward to a good grovel though and maybe with work, he grows more sincere and ditches that weird hero brother complex he seems to have around her.
I feel like Kallen was a bully when they were kids, and he's a jerk, and he deserves her anger... But I'm starting to feel a bit bad for him. He made a lot of awful choices when he was a child, but he made those decisions as a child. As an adult, he's trying to make amends, and she's fully justified in not forgiving him, but using and manipulating him is a decision she's making as an adult.