From Pluralpedia, the collaborative plurality dictionary
shell (adj.)
Applies toheadmates
OriginPsychiatric term

A shell is type of system member that normally only exists in OSDD-1A, is normally a host and is meant to hide the existence of other members from the outside world.

This is the member through which all other members act, fronting and possibly temporarily blending with the shell.

The non-shell members are not as distinct as in DID, though amnesia may exist between them.