Kouichi is fairly unappreciated pre digital world. Or at least he feels that way His grandmother dies and he's on a severe downward spiral.
After her death his moms worried a lot money and working and trying to support them but it's heavily implied that he can do all kinds of stuff with out her noticing ( Running around following Kouji all the time and trying to talk to him) Deep down he knows that she's trying to provide for them but emotionally he's neglected and he's the type of kid that puts others thoughts/feelings/needs before himself (think when Kouji kept asking what was wrong in the digital world) so his mother doesn't notice
This isn't something that develops over night and you can see it in the ep we see kouichis grandma tell him about Kouji He most likely developed like this and put others before himself due to his upbringing. Single parent and grand parent
We see his need to be noticed, not just as a person who's there but emotionally as well in the episode we see cherubimon vice brainwash him He seeks Kouji out hoping Kouji will emotionally relate to him and find a missing part of eachother but Kouji doesn't know about him He has the life Kouichi wished he and his mother had. house, dog, happy family, financial stability, the whole nine yards
He put all this emotional work into supporting his mother, being there for his grandmother (we don't see his mother when she dies with Kouichi at her side so we can infer that kouichis mother was working to ensure her family wouldn't go shelterless or with out food It's a tough thing for a kid to have to see someone wither away to nothing but ash ), searching out for his long lost twin, seemingly his other half- the one he shared a womb with. And what does he get?
His mother doesn't notice what he's going through because he puts up a strong front and thus his mother isn't providing emotional support to him he needs. Worst of all she lied to him all that time and he can't even tell her he knows. He's waiting for her to confess and when she doesn't it worsens their relationship. there's no trust anymore His grandmothers gone after sharing a horrible secret with him. His twin has a life that doesn't have room for him
So when he begs to be noticed and cries that's really heart breaking. It's that insecurity that cherubimon vice preys on and pushes Kouichi over the edge with I had a better conclusion but it's mobile and I'm sleepy