The influence of living environment. Yuyan Pei became a big sister because of its environment. The swallow has a "fighting fish" tattooed on his back, which is consistent with the title of the film. In a sense, it also implies that it is not enough to be protected if you want to survive. You must be strong enough to protect yourself and your friends. So she tattooed fighting fish, which is also a symbol of making herself stronger and more effective.