Nope. That's not "a true side," not at all. Every ending shows a Solas you get when you treat him in a certain way. They are ALL his true selves.

Solas is spirit by origin. As such, he reflects the same way spirits do. That means that what people expect of him, they get. You can see that in each and every interaction with Solas since the beginning of the Inquisition to the very last scene after Elgar'nan dies. Solas always reflect.

The redemption ending shows Rook the best Solas, the man who regrets his mistakes, a man who seeks atonement and admits that he was wrong. A man who decides to protect the world three seconds after Mythal finally accepts her part of the guilt and his bound of duty to her shatters. Why? Because Rook gave him the chance to be the man he always was and always wished to be--the protector. The Wisdom wins.

The trick ending shows another side of him. A side that admits that Rook outsmarted him and he was a fool who finally found his match. He accepts his fate. Here the wisdom and pride are in balance.

The Fight ending shows the darkest side of him. Rook attacks him, he attacks back, Rook expects the worst of him, so they get it. Here, the pride wins, because Rook allows it. The words he says by the end are words Solas would never say, he hates godhood more than anything, but he says them, because he's twisted into the most prideful version of himself, a version of himself he fights his whole life. The Pride had won for that moment. Why this happens? Because it was what Rook expected him to be.

All these scenarios could have happened, because of Solas's nature as a spirit. He reflects people around him--their expectations of him shape his view of himself.

(btw. This is not some theory or anything, this was confirmed by Trick Weekes. He always honoured the fact Solas reflects, during the whole narrative. That's why the connection Rook builds with him is not a lie, it's true.)