The Archangel - Psychological Description
Avacyn, The Archangel, is immensely benevolent towards humans. She was created to protect them, and she takes her job very seriously. She finds children easier to empathise with than adults - their innocence makes their mindset more similar to her own. Avacyn never smiles, despite her positive alignment, as she does not understand the point. It is believed that she can hear every prayer spoken in her home, Innistrad. She is also not above killing other angels if she decides it is the correct decision - she once killed another high-ranking angel (name unknown) and her entire flight (unit of angels) for allying with demons. It is also worth noting she fought the entire group alone. Avacyn has been described by other angels as one of the most powerful and confident of their race.
The Purifier - Psychological Description
After her corruption, Avacyn, now the Purifier, is a very different character. She believes humanity is the source of evil in the world - not the vampires, werewolves, or even the eldrazi abominations that she was made to defend against. She has commited gencide against humans in her beliefs, she did not even recognise the vampire who created her. Her fighting style greatly reflects her changed mindset: she is much more agressive and uses her speed and mobility to her advantage. She is short-sighted and hateful, focusing on the execution of humanity and ignoring the real threat.
Avacyn - Brief History
The vampire Sorin Markov created Avacyn to create balance between humanity and the various dark creatures that hunted them in Innistrad. By stopping either side from ever getting more powerful than the other, she prevented extinction on both sides of the fight. Though Sorin's method worked for a long time, it was eventually broken by the demon Griselbrand. He challenged Avacyn to battle, and after fighting for days, the only way Avacyn could stop Griselbrand was to trap them both in the Helvault, possibly forever. She did, removing the balance between humanity and the rest of Innistrad, throwing it into chaos once more. She was soon freed, however, by the necromancer Lilliana Vess, who wished to kill Griselbrand in order to free herself from the demonic pact that would cost her her soul should she fail. Releasing Avacyn involved releasing all of the demons in the Helvault, though the selfish Lilliana likely cared about neither the archangel nor the other demons - her focus was solely on Griselbrand. Upon Avacyn's return, part of her defence of humanity involved the cursemute - the transformation of many (willing) werewolves into wolfir, creatures like werewolves, but an unchanging mix of the two aspects rather than one sometimes and one others. In return for this, they were expected to protect humanity the way Avacyn did. A powerful and strange spell cast by Nahiri later planted a seed of suspicion in Avacyn's mind. Over the following months, she witnessed a number of foul acts commited by humanity which convinced her that humanity was unfit for her protection. After attempting (and in some cases, succeeding) to convince other angels of her beliefs, she made an attack on humanity, slaughtering hundreds before she was finally stopped.