Noober is male civilian who is seen as the the most irritating character in the game by many players.
You'll bump into him after crossing the bridge in Nashkel. He will then say something pointless to you every few seconds. No one would judge you if you killed him or polymorphed him into a squirrel. You will lose no reputation if you choose to take drastic measures.
Should you decide to let him live, he'll eventually get bored and leave, granting you an easy 400 XP.
Noober doesn't return in Baldur's Gate 2, but instead there is someone called Neeber. It's not known whether they are related, but Neeber has the same behavior and he will also reward the player for his/her patience.
In Baldur's Gate 2 Enhanced Edition its possible to inscribe Noobers name on the Scroll of Retribution during Dorn's Throne of Bhaal questline. Doing so will cause a random encounter to occur at some point in the future where both a planetar and Noober appear and the planetar declares Noober's crimes and then executes him.