... Fluffing stockholm syndrome, Suspension Bridge effect, & possibly mind control.
Yin Mei was once someone who fight against MC but lose. To survive being devoured, she became an accomplish. That alone should show that there is no good will between them (at least in the novel), only using each other. But the fluffing "care" MC has given to her and the fact that she stand too close to the abyss, now made her empathize with MC's Evil and even consider her fear as love to the person who have control over her life and death...
P.S. I'm just annoyed that she became a "love interest"... The plot and execution of it is pretty good. Not the best, but good enough to be enjoyed. But why going this route when you could make her into something like "a hidden blade in the dark"kind of character