Angelus appeared during Season Two.
DATA + APPEARANCES
-Name: Angelus, formerly Liam
-Monster Category: Vampire
-Key Relationships: Sired by Darla, Sire of Drusilla, Garner Sidle, Grandsire of Spike and Catherine
-Unique Attributes: Until recently (Spike), Angel was the only vampire known to have his human soul returned to him after becoming a vampire. Any moment of true happiness will remove it, reverting him back to his demonic self.
-Most Monstrous Moment: Killed Giles' girlfriend, Jenny Calendar in an effort to step up his torture towards Buffy.
-First Appearance (Comics): Buffy the Vampire Slayer: Ring of Fire GN (August 30, 2000)
-Other Appearances (Comics): Dark Horse Presents #141: "Cursed", Buffy the Vampire Slayer: Spike and Dru "Paint the Town Red", Buffy the Vampire Slayer: Angel: "Hollower" #1-3, Buffy the Vampire Slayer #29-30: "Past Lives", Buffy the Vampire Slayer #49: "Hellmouth to Mouth", Angel: The Curse #1 (all flashbacks)
-Current Status: His soul returned to him, Angel(us) relocated to Los Angeles as a champion for the Powers That Be.
One moment of true happiness. That's all it would take for the vampire, Angel to lose his soul and revert back to the murderous Angelus. During his time as Angelus, he killed the wrong girl -- a gypsy, and forever learned the price of that horrible mistake. The gypsy's family placed a curse on him as revenge, restoring his soul, so that he would have full memory of having done such hideous things and feeling anguish and guilt over having done them. For over a century, he went into hiding, living as a homeless person and taking blood from vermin, simply trying to forget. In 1996, the Powers That Be -- a mysterious force for good and order in the universe came to him, and gave him a new mission in life, to find redemption for the outrages against humanity he had committed, by fighting against the forces of darkness. Later on, came the Slayer, Buffy Summers, whom he instantly fell in love with. Though she knew he was a vampire, she could not deny her romantic feelings for him, and the two consumated their love - an action with many dire consequences.
The Gypsy curse upon Angel had a second facet of which Angel was unaware. Should he ever achieve even a single moment of perfect bliss, his human soul would leave him once again. Without hesitation, Angelus set about establishing himself as the patriarch of the " vampire family" that consisted of Drusilla, Spike and himself, all of whom were enemies of Buffy. He hated Buffy for making him feel human and in love, tormenting her and killed Jenny Calender, the girlfriend of Rupert Giles, Buffy's Watcher.
With Giles a wreck and Buffy not doing much better knowing the man she once loved was now a mass murderer, Angelus took advantage of their weakened states by gathering the armor of a samurai demon, Kelgor in exchange for ultimate power, which was promised to whomever revived the warrior demon. Angelus got more than he bargained for when Kelgor arose, betraying him and setting about to destroy Sunnydale on his own terms. In order to stp Kelgor, Angelus was forced to form an uneasy alliance with Buffy to take him down, later fighting Buffy herself in a battle for all the marbles. Buffy killed him, but the Powers That Be saw a greater purpose in him and brought him back with his soul intact. Knowing they could never share the passion they had without him reverting to Angelus, Angel left Buffy and Sunnydale, moving to Los Angeles to continue seeking atonement by helping the helpless.
POWERS + WEAKNESSES
Angelus has the strengths and weaknesses of the average vampire. His strength, though ill-defined, is far beyond that of normal vampires. He is also a skilled swordsman.
Nonetheless, Angel is not indestructible. He has all the vulnerabilities of a Buffyverse vampire: direct sunlight will cause combustion that would eventually kill him if he stays out in the sun's rays for more than a few seconds. A stake through the heart and decapitation will instantly kill him. In addition, certain objects of religious significance to Christians -- specifically Bibles, crucifixes, and holy water -- can burn his flesh. Also, Angel, like all Buffyverse vampires, can not enter a person's home unless he's invited by one of the occupants.