How to Recruit Tristian
Tristian can be found during the events at the Temple of the Elk .
A faithful Cleric of Sarenrae, Tristian traveled to the Stolen Lands to discover why the area seems so prone to harbor nefarious curses. While investigating such a curse in the Temple of the Elk, he was caught off guard by a particularly strong guardian, and would have perished but for the timely intervention of an adventurer seeking to bring the land under their domain. Afterwards, Tristian offered to support the adventurer who rescued him, as their goals seemed loosely connected.
The Cleric is a very versatile class, and for a fine demonstration of this all you have to do is look at the two Cleric companions in the game - Harrim and Tristian - although technically Tristian is an Ecclesitheurge, a Cleric subclass. While Harrim is a sturdy character capable of holding the line in melee combat (if being somewhat lackluster on offense), Tristian is more of a caster - a mage who happens to mostly cast divine spells. As such, do not expect to play the two similarly and have good results.
While Harrim will hold his own well enough as a caster, the same does not apply to Tristian's melee skills. First, he can't wear any armor unless you go out of your way to invest in an armor feat or pick up another class for him. Even if he could wear armor, his Strength is terrible, which is a fine argument to exclude him from melee combat entirely. In exchange, Tristian has more flexibility when preparing spells due to his Wisdom, more spells and high DCs on said spells. Give him a crossbow for when his spells aren't needed, keep him out of combat, and play him as you would any mage and he'll serve you just fine.
As for skills, he's got the typical Cleric selection of Knowledge (Arcana), Knowledge (World), Lore (Religion) and Persuasion. Since his Intelligence is nothing special, you'll be best served by improving the latter two.
If you've got plenty of front-line muscle and don't need a tertiary warrior, Tristian will serve as an excellent healer with a smattering of fire spells to bolster his offensive output, for good measure. Plus, since he possesses the "Selective Channel" ability, he can channel positive energy without harming Jaethal, so the two work well in a party together, despite their animosity.