His fate depends on your choices BEFORE Lost Izalith, not in Izalith.
If you're in Izalith, and you have already talked to Solaire at the bonfire after centipede demon, the path is locked in.
So don't go into Izalith, don't talk to him after the centipede demon.
Deal with the bug first!

No idea if this has been mentioned anywhere, but the exploit to kill the bug through the Lost Izalith shortcut wall seems to have been patched in Remastered. I tried Poison Mist, Dragon King Greataxe special, and Drake Sword special, and none of them worked.

*dies to headcrabs*

This dude is best npc in all of souls history becuase of the journey:

When you first meet him, he's like a shining beacon of hope that also kicks ***. Then he keeps meeting you and gettng stronger, but losing faith in his sun. You're the one that gives him hope by the end of the game, and considering he actually can solo gwyn on ng, it really does show how strong he got when he finally linked the flame and became the ultimate sunbro.

Damn you Solaire, it was just missclick [used 100K souls for removing sins]

\`[T]/

Praise the Sun

solaire of astora would 100% ditch the sun and gwyn and boring age of fire and the chosen undead for a PKCS

he is not a meta trash casul

he knows that the PKCS IS OP
be like solaire

Use PKCS

Is there a way to get his set and get him for the endboss in one playthrough

I believe, solaire is miquella. Renna is the doll in Bloodborne followed by a beast.