"What you call self is nothing more than a mass to cover your own being."
I have several different interpretations of this statement--I understand the entire conversation except for this one part. Here's my theories on what this means:
1) Raiden does have a core personality "self." However, he dislikes this self, so he tricks himself into thinking that his actual personality is something else, something to cover his "own being," his own self.
The second theory ties into the line after that one, which is this: "Another possibility is that self is something you use to preserve those positive emotions you occasionally feel."
At times, Raiden feels positive emotions, either from external or internal causes. If Raiden feels a positive emotion stemming from, for example, an outside event, depending on his current state of mind, he may or may not feel guilt over having that positive emotion. If he does feel guilt over having that positive emotion--possibly arising from a discrepancy between the positive emotion and his current beliefs--he would, in order to preserve the positive emotion, create a new "self" for himself so that there is no dissonance between the new self and the positive emotion that he is trying to preserve.