You're the other half of her soul.

Darla attends an elite prep boarding school for extraordinary individuals: for the spawn of monsters and/or aliens, for crazed apprentices, mad interns, and budding would-be world conquerors who might be too young to drink yet. Mad science and madder magic are staples of classwork there. Status at the school - at least the status Darla cares about most - is determined by one on one duels. Darla is in 4th place at the school and frustrated she's plateaued, unable to move upward no matter what she does. But she realized there's a loophole in the rules; if she gains an ally that's a part of her, the duel is still technically one on one. So she created a ritual to give up half her soul to summon someone or something to help her - you!