Jar Bairn is a character that his questline is intercepted by Diallos and Alexanders so you should do this questline. Also he's cute. And when I say JJ I mean Jar-Bairn because it's long to write it's name.
Alexanders questline
Diallos questline
You can find JJ at Jarburg, south of the Carian Study Hall place and you will find him sitting down on a ledge (As shown on the picture). Talk to him and always pick the nice option to proggress it's quest. Reload the place with a Site of Grace and talk to JJ again. It tells you to pick rare flowers in jarburg (You do not have to pick up all the flowers). After that reload the place and talk to JJ again. Now JJ will talk about his uncle Alexander, Just reload the place again.
Talk to JJ and now he will be talking about some Poucher coming jarburg or smth and Reload The Area AGAIN and talk to JJ, this will make Diallos appear in Jarburg. Go to Diallos and exhaust his dialouge and just leave Jarburg for some time
After that go back to Jar Burg and Diallos will be dead, along with others jars. Talk to Diallos until he repeats whats he's saying then talk to JJ. RELOAD the area then JJ will be sitting on the staircase of some house (Stoop).
Before continuing with JJs questline you have to do Diallos and Alexanders questline
Alexanders questlineDiallos questline
Go to Crumbling Farum Azula and you will meet Alexander here that wants to challenge you to a duel. After defeating him you will be given Alexander's Innards. Return to Jarburg and give these to JJ and RELOAD the area to get the Companion Jar Talisman from where JJ had been sitting