Backup is the loyal dog, a pit bull, owned by Keith and Veronica Mars. Veronica often takes him with her to protect her from unsavory types, although Backup refused to be unfriendly toward Logan Echolls when Veronica suspected him of killing Lilly Kane.
Veronica: I'll be at home. With the only sane member of the Mars family. Keith: The one who eats from the garbage and keeps bringing me dead birds? Veronica: That's the one.
The dog who portrayed Backup changed after a few episodes: initially played by the white and brown American bulldog Gordo, he was replaced with peachy-cream colored pit bull named Lefty.