OOC: he has a real name,
but giving it away would greatly increase the ability of others to harm him, if you've played Planescape Torment you're familiar with this concept, also many demons cannot be summoned unless you know their true name, hiding his own, even from his most trusted advisors ensures his power base is not threatened.
you can call him Morhde Sihthe (i think that' show I spelled it the first time)
As far as playing a bad guy in addition to your main character, PM me first with what you want him to do/be and keep their power levels reasonable. also be vague as to what they are doing in the background as there is no real way for the main party to understand exactly what is going on with their foe (remember Balagos' treachery?)