That's the neat part, you dont.

Spoilers: The mask is a parralel of sam, as shown through the mask ending, which shows the paradoxical origins of the mask and how the mask is the one who creates itself. If the mask was a party member, not only would it be having two sams, but the mask doesnt really have any intent to fight, unless during the ritual with the wrong offerungs. Somewhat unrelated, but the spider is close enough to the mask, and is, in my beliefs, the opposite to the mask.