So how does Chisaki handle emotional pain? Both inflicting and being inflicted. You said he's good with physical, so I was curios about this one.
cracks fingers Thank you so much for the question, friendo!! I love talking about Kai’s mind. I’m a Psychology major so I go overboard with it sometimes, but here it goes!
Kai is used to emotional pain, though not to the extent he is to physical, and in no way he can withstand it as much. Emotionally atacking him is the most likely way to hurt him with lasting effects. He can heal his physical scars just fine, and he can rebuild and restart most of his plans, but, as I wrote in a thread with my Hari, Chisaki cannot Overhaul memories. And that is one of his biggest issues.
He absolutely hates that he cannot let go of his feelings nor his past traumas. A good part of his overrational behavior is due to his attempts to distance himself from his emotions, to protect himself from what he sees as a weakness. He hates feeling strong emotions, and is unaware of how much they influence his decision-making (such as rebuilding the yakuza for his loyalty to the Boss). Any situation that makes him feel out of control, useless, weak or humilliated are particularly bad to him, for reminding him of his old caretaker.
Chisaki has fully internalized several toxic ideas created by emotional abuse, such as that he is a cursed, unwanted being due to his quirk. This was used in the past to emotionally manipulate him and keep him compliant, exactly the same thing he does to Eri now. Kai is not emotionally mature or empathic enough to break the abuse cycle, which is another tragic layer to his character. To him, any evil things he does is just an extension of his original sin, which is Overhaul, and he was already condemned from birth for carrying it. It is one of the reasons he chose the quirk name as his villain name; he was born bearing this curse in his veins and he will die with it.
As for emotionally abusing others, besides his whole relationship with Eri (which takes a lot of influences), Kai has no issues with emotionally manipulating and abusing others if it will get him closer to his goal. Like I said, he believes he is cursed by nature, and has already given up on redemption or humanity in order to achieve his main objective. Then there is no need to hold back.
That said, it is difficult to hurt him emotionally if you are not already aware of his existing issues, and he hides those well precisely to not feel vulnerable. It is not impossible, but you need to know where to hit him!