Mystique

You may have Mystique enter the battlefield as a
copy of another creature on the battlefield,
except it has "{u}{u}: Exile this creature, then
return it to the battlefield. Activate only as a
sorcery."
{u}{u}: Exile Mystique, then return it to the
battlefield. Activate only as a sorcery.
"Who am I? That, my dear, is an excellent
question."