This character is an ex-girlfriend.
In social relationships, an ex (plural is exes) is someone with whom a person was once associated. As a prefix, ex- can refer to a variety of different relationships; for example, one might refer to a music group's ex-guitarist, or someone's ex-friend. When used alone, ex as a noun is assumed to refer to a former sexual or romantic partner, especially a former spouse. This often has a derogatory tinge, especially if it refers to unrequited feelings.