In the film The Warriors, Mercy is a whore--or not a whore--who originally is with the low-tier gang The Orphans. After The Orphans and The Warriors cross paths, Mercy leaves The Orphans and follows The Warriors. She is caught by Swan, their warchief, who threatens to have the gang "pull a train" on her. However, The Warriors instead let her tag along with them.
Swan and Mercy eventually become separated from the rest of The Warriors, and they slowly develop a romantic relationship. Their romance is strained, however, as Swan does little to hide his disapproval of her lifestyle.
By the end of the film Swan and Mercy appear to come to terms with one another, and it looks as though their romance will continue.