[riderkid]

Originally Posted by no-name

Legend tells of an ancient artifact that can grant a woman's wish...

THE SKULL HEART

However, if she is impure of heart
her wish will be corrupted and she will be turned into
an immensely power, twisted being of destruction...

THE SKULLGIRL

A new Skullgirl has risen; who will defeat
her and claim the Skull Heart?

-Skullgirls Opening Text

I've yet to complete the game with every character in Arcade and Story Mode (0 Arcade/2 Story), but it was my understanding from the intro alone that it takes a completely pure heart to make a wish on the Skull Heart without turning into a Skullgirl. Considering the endings I've seen so far, wishing on the Skull Heart may require a messianic-level of purity or higher, so even the nicest person can become a Skullgirl if their heart possesses even the slightest hint of darkness. Good luck finding someone who can meet THOSE requirements in that world.

...Kasumi Tendo?