From what I've read, it's a fairly involved process to keep him around until then, with the only benefit being help for the final boss. This is my first playthrough. Is it worth the effort? Does his summon make the final boss easier?
Kill the sunlight maggot before killing centipede demon and you've got Solaire for the last boss. Literally that simple.
He is not worth it in my opinion. Gwyn's attacks have a wide arc so having Solaire take the aggro doesn't help much because Gwyn's attacks hit you when you are behind him and also Solaire has a very small amount of health which depletes very fast. Solaire deals lightning damage which Gwyn happens to be resistant to. Keep in mind that Gwyn will probably gain health if you summon Solaire. Parrying is way harder with Solaire when Gwyn is swapping target and his sword will hit you even if you're behind him. You can't parry an attack from behind someone.
(\ (O_O) /) Praise the sun! http://www.youtube.com/watch?v=b0O1sH1N6n4