i've drawn heather's shadow form just to show how it looks like, her abilities in shadow form could be making portals and Big hands coming from the portals which she could use to help her allies or fight her enemies, and the portals would also work to teleport to other places, but she could even have more abilities, and also whenever she gets in her shadow form, she becomes more lighter

she mostly uses her shadow form in battle or stealth missions, but she also uses it for pranking and for fun

How she could transform and how she could return back to human form

Transformation:

1: She gets consumed by her shadow

2: Her shadow stays in the floor

3: She rises up from her shadow as a void-like human

Returning back to human form:

1: When she goes to a very lighted up place (for example, a place where its so sunny), she starts sweating black liquid

2: as soon as she sweats black liquid, the liquid could go to the floor and probably dissolve, transforming back to her normal shadow and herself to her human form

(FUN FACT: Heather also has a cosmic form, as an light alternative to her shadow form)