Episode 39 of "Angel"
s02e17 Disharmony

Angel Investigations may be together again, but that doesn't mean the family is ready to add a new sister. While the crew tracks down a cult of vampires led by a former motivational speaker with a wicked pyramid scheme, Cordelia gets a surprise visit from her old high-school buddy Harmony. Unaware her flaky pal's a bloodsucker, Cordy mistakes Harm's strange behavior for lesbianism---until the truth comes out, so to speak, in a call to Willow. Much to the dismay of her co-workers, the ever loyal Cordelia resolves to give her undead friend a chance at fighting for the good guys.