The black cat is Wang Long Chizuru's familiar. In Ellen's route, it traps the party in a Kimon Tonkou formation inside Sumaru TV. The black cat appears before the party many times in the guise of Makoto Kuwahara and the Boy with Earring before having its true identity exposed and fighting the party.
In its boss battle in Persona 2: Eternal Punishment, attempting to retreat will simply display an error message reading "!!!!!!!!!!!!," unlike most bosses who have special dialogue for retreat attempts.