Ego
Awareness in the service of fear. Not the same thing as having a self.
My Big TOE's definition is one line: ego expresses awareness in service to fear. Having a sense of self is described as both necessary and good — the ego is what happens when that self-awareness gets recruited by something you are afraid of.
It shows up behaviourally. The glossary lists refusing to help others, preventing their success, and taking pleasure in their misfortune as its expressions. It operates at the intellectual level and at the being level both.
The useful property, for practice, is that ego is far easier to spot than fear. It announces itself as a bad feeling. That makes it the doorway.
Where this gets misread
That the goal is to have no sense of self. That is not what is being described, and pursuing it produces a fairly recognisable kind of spiritual damage.