Changcheng's parents were killed in a car accident when he was young. His grandmother raised him, and when she died, his uncle (Guo Ying) and aunt (Yu Jinlan) took him in. His uncle works in the government as vice-minister.
He joins the SID as an intern in episode 1 at the age of 24. He says that "someone" told him to report there for duty and Zhao Yunlan assumes it's his uncle until he later learns that the uncle didn't want him to work with the SID because it was too dangerous.
At first, he is intimidated by Chu Shuzhi, but over the course of the drama, they evolve into primary partners/family/lovers.
The SID team don't treat him very nicely at first, they Halloween-scare him, and dangle him perilously out of a crime-scene window for no obvious reason. Zhao Yunlan repeatedly calls him cowardly. None of that deters him, and it is patently untrue: In episode 1, after he escorts Li Qian's grandmother home, the shadow man comes looking for the Longevity Dial, and Guo Changcheng, although understandably terrified, hurls an apple at him. At the end of the episode, he questions the procedure for dealing with Dixing detainees in front of the awe-inspiring Black-Cloaked Envoy. And by episode 2, he's doing his utmost to protect Shen Wei and Li Qian at the hospital. Even the bad guy looks bemused and/or moved by his tenacity.
Guo Changcheng embodies huge amount of goodness and compassion. He's respectful to strangers. He has a strong moral compass. He's really kind. (The peptalk he gives Ji Xiaobai in episode 5, when Zhou Weiwei is missing, is both super-dorky and effective.)
He doesn't let his anxiety, awkwardness or fear get in the way when it counts. (He stands up to Zhao Xinci!)
He's goofy and innocent. He keeps a diary and a list of Zhao Yunlan's sayings, and tries to make sense of and learn from everything that happens.
Zhao Yunlan gives him a custom-made weapon: a zapper for turning fear into electricity.
In episode 5, after the water kidnapping case when, faced with Butler Wu dying and his son's grief, everyone in the SID stands around feeling helpless. Zhao Yunlan looks to Lin Jing, who shakes his head: there's nothing they can do.
It's Guo Changcheng who goes and fetches the Longevity Dial from its case. He's the one who thinks outside the box and summons the determination to risk and to sacrifice. In that moment, he is more heroic than any of them, and Zhao Yunlan immediately learns from Changcheng and takes over, setting a new course for the SID and paving the way for a closer relationship with the Black-Cloaked Envoy. Guo Changcheng's actions -- and his belief in the SID as a force for good -- are pivotal.
- We... we have the spirit. We don't care if we have to sacrifice ourselves to protect the people around us. Chief Zhao, the Deputy, Chu-ge, Hong-jie... even Sang Zan who joined us recently. They all have this kind of spirit. They are my role models. Although I'm a coward, I still know that even if I hide somewhere, how long can that last? If danger is going to come, someone has to stand in front of everyone else. (ep 13)
- Uncle, you're right, I'm useless, but at the SID everyone looks after me. Although I always need help, they still take the trouble to take me on cases, teach me fighting skills, and when there is danger, they protect me. It's the SID people who give me courage and confidence. It's them who help me find my own value and my future path.(ep 14)