Stan is actually an android. Chaz knows how androids work, and it seems he's a specialist. He was able to 'erase' Stan's memories of his ex-girlfriend, so he's like an android programmer. That's why Donen wants to keep him around. If Stan becomes messed up again, Chaz will always be on-call.
Here's an ironic fact. In the original play, Donen actually pushes Stan, who is very introverted, to go out with Vivi.
Unfortunately, I didn't find out who Yasujiro is.
Cheers!
Sorry, I just realized that I got the names switched around. So Chaz wants to keep Donen around because Donen is the android expert. Stan is an android and also Chaz's brother. I'm really sorry if I just confused people further.
Can someone explain to me when chaz saying to donen that stan needs him. Like how does stan need donen?