Well, as I see it, you have to give her (and anyone/anything she touches) the ability to be unaffected by physical attacks and the ability to move through borders (except rifts, pits and the like) as if they're not there, and I don't think it makes sense to say she uses Discipline to do it - the only conscious "effort" she makes to use her power is switching it on, then she walks (Athletics) through borders and ignores physical attacks (no roll). Beside that, you have to give her bonuses to all Stealth trappings and to Infiltration, also to blocks, maneuvers and possibly to attacks from inside a solid/with objects (maybe you want that to work as "forcing" the target to use a specific skill when defending - read Might/Athletics - or maybe as attacks bypassing armor).
Also, if you go the movies route, you have a form of Time Travel on your hands. If you go the comics route, you have teleportation, flight, Fists, Weapons and Scholarship stunts, and a pet dragon. Something like a touch-based Mana Static and some kind of defense/resistance to supernatural mental attacks, too. But really, this just means she would have a high refresh level, and in the X-Men universe, that wouldn't be out of place. It's just the Phasing that's broken, in my opinion. She can walk up to someone with a pencil, stick it into them while completely phased, let it go and have it return to its normal solid form now inside their body. She can do the same but with her allies attacks, and actually, now that I think about it, she can also make her enemies' attacks pass through her body and her allies' by simply touching them (and still remain phased herself). She can go anywhere in a building, easily out of reach of anyone with access to her catch for example, or to spy, prepare ambushes, steal stuff, etc.
I honestly think it's broken anywhere, but it is especially so in what amounts as a turn-based system, unless you give it limitations that, canonically, it doesn't have.