He's indeed going to be in the sequel, there's a character with his name. But I read a commentary that in Forever…
I think you're thinking about his character as a rational person. He is terminally ill. That's part of what makes him irrational. He feels like life is unfair and like Anlly above said, Shiyi was his only light. She should have been on his side and she wasn't. Shiyi that cannot go 5 minutes without thinking about ZSC. So he just wanted to burn everything to the ground. He was definitely not stable.
At epi 14 currently. I am not clear about Shiyi's a purpose/ goal - like what is her raison d'etre at this stage?…
You missed the point of why he was asked to mentor her. It was a political move by her Uncle to make sure the Cui's had ties to the Nanchen Prince since the Palace was playing games with her marriage. That is, not putting all of ones eggs into a single basket. The mentoring was just an excuse. No one actually expected serious mentoring. If that makes sense
The Gods gave him someone he fell in love with and he took care of her without calling this "Love". This is beautiful.😔