Woo boy, this man is complicated. We see Cheng attempt to murder his adopted brother (Wei Wuxian) in the very first episode of the Untamed (not a spoiler), but we'll find out why the rage has built in him throughout the series. He comes by his frustrated, angry nature honestly - his mom was a raging bitch too. Both are extremely competent and possibly even more uncompromising than the Lan Clan. Doesn't like Lan Wangji, has complex feelings towards Wei Wuxian. Loves his sister and dogs. Don't cross him or come across his radar - he won't forgive you.
"A violet-clothed youth approached in confident steps, his jianxiu robe flowing smoothly and his hand pressing on the hilt of his sword. A silver bell hung by his waist, although it made no sound as he walked. The young man had thin brows and almond eyes. His features were handsome in a sharp way, and his eyes held a composed vigor, with a slight intention of attack, appearing to be two bolts of lightning as he stared... Even his posture emitted an air of arrogance and overconfidence."
Jiang Cheng distracted the guards after Wei Wuxian and, as a result, was captured by the Wen Clan and lost his golden core. He never told Wei Wuxian about it.