Jan's Companion Quest - Baldur's Gate 2
Jan's Companion Quest begins about 15 days after he first joins your party. You'll be approached by a man named Beeloo who will tell Jan that Lissa is sick and he needs to return home to see her. You can find Jan's home in the Slums District of Athkatla, it's on the eastern side of the map.
Note: During this quest Jan will leave your party. If you take too long to answer the summons to return home then Jan will also leave your party for this. You have a few days before he starts to get impatient.
After the family reunion you'll be tasked with going into the basement to speak with Uncle Gerhardt, a popular character in many of Jan's stories. You can get through most of his dialogue by spamming 1, ask him where you can find someone to cure Lissa and he'll send you to the Jysstev Estate in the Government District.
The quest says speak to Jan again but you don't actually have to, head to the Jysstev Estate in the Government District and speak to Lady Jysstev about "The Hidden" to update your quest. Lady Jysstev will direct you to the Slums Sewers beneath the Copper Coronet which is where you'll find "The Hidden".
You can find the person we're looking for in the middle of the zone standing alongside the southern portion of the river. He is named simply, Hidden and all you have to do is talk to him and agree to help him with his problem. The next part of this quest involves a bit of running back and forth so I will break it down in a list format to make it easier.
1. Visit the Sea's Bounty Tavern in Docks District
2. Speak to The Thumb at the tavern and tell him you are a seeker
3. Head to the Five Flagons Inn in Bridge District
4. On the 2nd floor you'll find 2 Githyanki in a room who will attack you immediately after you enter
5. With the Githyanki defeated return to Hidden in the Slums Sewers and speak with him
After speaking with the 'Hidden' in the Slums Sewers all that's left is to return to Jan's home and talk to him again. Vaelag will visit the house during this conversation and you'll get to meet him downstairs. It's up to you, but I typically attack Vaelag after this dialogue and kill him. There will be no reputation loss or penalty for doing this, but it sure does feel good. I got your back Jan!
Once Vaelag leaves you'll get some dialogue with Jan - agree to support him when the time comes and he will rejoin your party. The quest ends here and there never is any follow up (unless you're using mods). Also if you took the same route I did and eliminated Vaelag there can't be any follow up anyway.