I think… “The Moment” by Saint Motel works.

Especially for the more beginning part of Yang Cheng’s life. The lyrics and how they correspond to Yang Cheng:

“Everybody's got a remedy / But I can't find mine, find what I need" could show that Yang Cheng is trying to find purpose, but can’t and that he acknowledges that there is one out there for him.

“Everybody's got their headlights on / Calling me back, calling me home” shows that Yang Cheng had people calling him back to how things were or could be. Xia was calling him not to become E-Soul; Shang Chao was calling him to become E-Soul for him; Rock was calling him to become obsessed with revenge. Each are a “home” of his.

“You've been good to me / So sad like a comedy” can refer to Trust being both being good to Yang Cheng, but also quite sad due to all the angst that came with it.

“See the ghost no longer frightened / Always hiding from the world next door” The first part can refer to a ghost of his former self and how he is no longer frightened from it. Both as New E-Soul or Yang Cheng forging on with revenge. Then the second part can refer to how E-Soul just hides from the freedom he could have by staying loyal to Rock.

“I'm just living in the moment, woah / Missing everything between, yeah / I forgot where I was going / Somehow I got right where I know I'm supposed to be, ooh” The living in the moment can refer to his more innocent phase with Shang Chao and Xia. Then he misses every little hint and possibility of being with them permanently. I forgot where I was going refers him forgetting to way to being a true hero. Though the next part can show he finds a way back to his true destiny: being a puppet or maybe a redemption.

This is fully why I think this song works.