Just unlocked.
Talk to him in Jeuno and trade him 100,000 gil, he then asks for an elixir or high-elixir. Not sure how this effects his interaction as a trust, but definitely related.
When he's summoned he makes mention of the donation and the elixir.
Uses a variety of Mix Potion spells:
Max Potion: 500hp-700hp cure
Mix: Dragon Shield - Magic Defense bonus buff for ~1min
Mix: Panacea-1 - a panacea, removed a debuff
Mix: Dark Potion - did 666 damage to enemy target
Mix: Samson's Strenght - all attr. stat bonus (+10~, 60s duration)
Mix: Guard Drink - protect/shell for ~4min (~450 defence potency)
Mix: Life water - Aoe Regen (20hp/tick)