Witch Beatrice

Witch Beatrice is a summonable character in Dark Souls. She is a spell-caster providing some ranged magic support for the player.

Location
Beatrice appears as a summon sign under the staircase leading up to the Moonlight Butterfly (only if the player has had hollowing reversed and not beaten Great Wolf Sif) in the Darkroot Garden.

Her summon sign is later found before the Four Kings fight in the Abyss.

Plot
Aside from being a summonable NPC for the boss fights labeled above, Witch Beatrice has no obvious contribution to the game through either story or gameplay, although her attire can be obtained.

After summoning her for either of her fights, her robes and catalyst may be found on a corpse at the entrance of the New Londo Ruins from the Valley of the Drakes.

Backstory
Witch Beatrice is mentioned to be a catalyst-user that did not hail from one of the dragon schools, setting her apart from other casters. She travelled to the Abyss but succumbed in her journey. That the player discovers her items at New Londo confirms this, and also suggests that she was either killed by the drakes guarding that entrance before she could enter the Abyss, or that she was seeking to escape from the Abyss only to die of her wounds once she reached the exit.