Ghost Girl (Lili Stephens) is a fictional mutant character, a superhero in the Marvel Comics universe. She is a former member of the superhero team Alpha Flight. Department H call her a "Legacy" case, but they never explained what that means.
Powers and abilities

Ghost Girl possesses the ability to "phase" or literally pass through solid matter by passing her atoms through the spaces between the atoms of the object through which she is moving. Being intangible she becomes invulnerable to physical attacks. It's unknown if she has the skill with phasing as Shadowcat does. Her powers were never explained; it may be assumed she uses the same process Shadowcat does.

She can also use her intangible body to create gateways through solid objects for others to use. She has created pathways for Puck and Flex to pass through her, and doing so tickles her.