I, as in personality (3)
I believe the answer is:
ego
'i as in personality' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ego that I've seen before include "Self-esteem, sense of self" , "Person's sense of self" , "One's self-image" , "Inner self" , "Selfishness" .)