On your way to the Moon Temple, you'll be flying through the Cliffs of Erin - lots of little floating islands in the sky. Tristis is sitting on the far left of the one of these floating islands; he's in a blue jester suit. Speak to him to have him join you. This will also complete his sister's request if you don't have the trophy for the Lemurian requests. Find him in the upper left of the general area; look for balloons.