Helping Tiris - Baldur's Gate 2 Side Quest
In order to begin the Helping Tiris in Trademeet side quest you'll need to have completed both of the main quests involving Trademeet, Animal Troubles and Dealing with the Trademeet Genies. Completing both of these quests will name you the Heroes of Trademeet which is one of the 2 requirements to begin this quest. The other requirement is you'll have to complete Solve the Skinner Murders in Bridge District side quest which, as the name suggests, takes place in the Bridge District of Athkatla.
Assuming that you have completed all of the prerequisites you'll be approached by Jenia in Trademeet who will ask you to visit Tiris' Home and offer him your aid. Tiris will update your quest when you talk to him, after which you'll want to head to the southwest exit of Trademeet which is where you'll run into Darsidian Moor who claims to be hunting the same "skinless creature" that you are (pictured below).
There will be some dialogue with Darsidian Moor during which he'll ask you to help him at a pavilion in the southeast portion of Trademeet this time. It's the same part of Trademeet that you started at your first time entering the town. Use my screenshot below if you need help finding any of the locations involved in this quest.
When you arrive at this location Darsidian Moor and Raissa will tell you that they've knocked out the creature and that you need to deal with killing blow quickly. The "creature" they have knocked out is actually the real Raissa just wearing Rejiek's skin. At this point in time the quest technically forks off, if you deal the killing blow here then you'll get a 20k EXP reward when you return to Tiris.
Alternatively, you can talk to "Rejiek" (the body on the ground) to learn it is actually Raissa which will provoke both Darsidian Moor and the real Rejiek to attack you. Once the fight is over Raissa will put on her real skin and tell you that she needs Restoration cast on her. If you don't have a Priest who can do this then visit the Temple of Waukeen nearby and buy a Lesser Restoration scroll to use on Raissa.
Using Restoration on Raissa will get you the best outcome of 30,000 EXP and +1 Reputation when you return to Tiris' House and talk to his parents. Refusing to cast Restoration on Raissa will result in the worst outcome, no EXP at all and when you return to Tiris' House you'll learn from his parents that Raissa left with Tiris and is now gone.
Last but not least you have the option of refusing to cast Restoration on Raissa and instead showing her the sharp end of your sword. This will get you a total of 20k EXP when you return to Tiris and speak with him.