A savior complex (Christ complex or messiah complex) is a state of mind in which a person believes they are responsible for coming to the aid of others that need help. The character diverts a lot of their energy to seeking out those they feel need their assistance, but often it is just an outlet to escape their own problems.
A "Savioir Complex" sometimes manifests by getting into relationships with damaged people. Here, Phoebe Bridgers sings of a difficult relationship with a needy person who is sucking the life out of her.Baby, you're a vampire
You want blood and I promised
I'm a bad liar
With a savior complex
Bridgers explained to Stereogum
the song is about dating someone who hates themselves. "'Savior Complex' is you get there, you're with the person, and you realize that every other person they've been with before has gone through the exact same thing of trying to break down walls. Being with somebody who hates themselves is hard."