Satan is certainly not the ego, which is the (partially) self-regarding and self-controlling aspect of your reduced personality. Satan is AN ego, you could say, one that is aggressively self-centred and tries to inspire you in the same direction, cutting your own ego off from its loving, friendly, open-minded, and receptive impulses. You could see Satan as a sort of vicious and brilliantly stupid cartoon ego that infects your ego and turns it into that sort of anime dragon that can only use its powers to feed itself the illusion of greater power.

I don't pretend to understand or verify Satan as an entity, but I watch out for that sort of thing. And pray against it. Deliver us from evil.