Forget about a deep psychological breakdown. The premise of 'City Hunter' is shockingly simple: Ryo wants to honor his partner's dying wish to look after his sister. That's it. That's the core. The problem is, the sister, Kaori, thinks Ryo killed her brother. So you get this bizarre, hilarious, and sometimes surprisingly touching dynamic where Ryo is trying to fulfill this promise while constantly being smashed over the head with a giant hammer by the very person he's sworn to protect. It's not some grand revenge quest or a mission to cleanse the city. His motivation is literally a dumb promise to a dead friend, twisted into a daily slapstick routine. The real drive for most episodes is just... work. He's a sweeper, he takes jobs. The 'city hunter' part is his job motivation: money and women. The Kaori situation is his personal, messed-up anchor. They barely explain it beyond that, and honestly, it works because it's so straightforward. The complexity comes from how those two threads—the professional horn-dog and the guilt-ridden protector—constantly clash.
You see it in every case. He'll be lecherously chasing a female client, then switch to deadly serious the second Kaori is genuinely threatened. The show doesn't need long monologues about his pain; you see it in his eyes when he thinks she's not looking. The manga goes a bit deeper into his past with the mercenary group, but even then, it's less about explaining his motivation and more about showing why he's so emotionally stunted. The 'sinopsis' gives you the setup, but the character's actions over hundreds of chapters and episodes flesh out a guy who uses his goofy facade to hide a lot of unresolved loyalty and grief. He's motivated by a debt he can never truly repay, and that's kind of tragic when you think about it.