General obsession with the sun. He painted it on his armor and sheild personally. Search for his own personal "sun" which leads him to go crazy.
He's one the only 2 NPC's that use thrown lightning. (Other being Ornstien who is not the firstborn for many reasons.)
His affiliation with the firstborn's covenant.
Sunlight Medal description: The symbol represents Lord Gwyn's firstborn,
who lost his deity stats and was expunged from the annals. But the old God of War still watches closely over his warriors." Solaire literally watches over members of the sunlight covenant via boss fight summons.
If you kill him the sunlight alter fails to function giving credibility to the idea he is the source of the covenant.
He gives a really straight forward clue in dialog: "Oh, hello there. I will stay behind, to gaze at the sun. The sun is a wondrous body. Like a magnificent father! If only I could be so grossly incandescent!"