You gain the ability to transform into any one fictional character from an official source (that you recognize as fiction), as well as gaining all their abilities, memories, and skills. You have their abilities, memories, and skills even when you're not transformed, and can summon any item significant to the character at will in either form. Any character chosen that is a planet-destroyer will have their abilities weakened so that they're hard pressed to destroy a planet. Don't choose an Eldritch Abomination, you will go insane. You gain their weaknesses as well.
You can only have a single spirit active at once but can switch between them at will.