He built a castle for him to house this crow, which was nominally a canary but failed in it. Si'er has been waiting, waiting for himself to be strong enough and Qin Muqing to be weak enough, that is when he can break out of the prison and regain his freedom.
Before that, Si'er had met many people, as well as things that looked like humans but lived like animals to survive in this troubled world. In this cage, he saw the whole of Shanghai. Countless people died in the disaster, but the most damned people were still living intact in a broken castle on a broken mountain.
"He was like a crow flying from afar on the tragic battlefield after the war. He saw hope and depressing despair." Si'er said, with only sadness and a circling goose in his eyes.