After the Big Head died, the cowboy hat revealed its 'true form,' and the Ancient City reverted from half-new to its dilapidated state. However, Ou Chen remained unchanged, which at least indicated he was still alive.
Both the cowboy hat and Gu mentioned that the Evil Spirit could freely alter the city's architectural structure, such as instantly turning into a real underground passage. So perhaps Ou Chen appearing in the 'wall' was merely the Evil Spirit imprisoning him in a 'cell.'
Now that the Big Head is dead, Ou Chen walking out of the wall can be considered a regained freedom.
"Are you coming with me?" I asked. He sounded convincing, but those who only talk without taking action are mostly unreliable.
Even if he can't enter the core to destroy it, he can at least lead the way. Without a guide, I might waste a lot of unnecessary time on the road.
