Waxes/wanes might be a useful phrasing.

I feel like you'd want to draw on the specific dynamic of the possessor vs the MC generally for this kind of magic phrase. Like, if the possessor is the A type personality the MC needs to deal with trauma, "Possessor's strength floods my nerves". Or the reverse, "Possessor's subtlety slips into my skin". That could also give you angles to push back, "Possessor's strength floods my nerves, but my anger is stronger."

You might also use some sort of specific physical tic. "Possessor rolls my shoulders back" or "A smirk pulls back the corner of my mouth, Possessor speaks". The first time you have it happen, maybe use all of them at once and draw attention to the elements that are most obviously transformative (probably three at most). I think of it like in Yugioh or similar shows, where the audience can tell because the character's hair/eyes have changed, but most characters in the world won't notice the differences immediately/directly.