Keldorn

Keldorn is a Lawful Good Inquisitor (a specialist paladin class) who can be recruited in BG2. He makes a good frontline melee character, although as a paladin he can only add two points into any weapon specialisation, as opposed to a fighter who can have up to five points. However, unless your player character is a paladin, Keldorn is the only party member who can wield Carsomyr, a powerful two-handed sword which offers 50% magic resistance and casts dispel magic on each hit, making Keldorn a powerful anti-mage character. He can also speed-cast True Sight several times a day, which is useful for removing illusions such as Invisibility and Mirror Image.

Baldur's Gate 1

Keldorn does not appear in BG1.

Siege of Dragonspear

Keldorn does not appear in SoD.

Baldur's Gate 2

Keldorn can be found in the sewers of the Temple district at the beginning of chapter 2, once you have accepted the Unseeing Eye quest (it's difficult not to as you will be approached about it before you reach the entrance to the sewers). You don't have to complete the quest to keep Keldorn in your party, and unlike some other characters he will not complain if you take a long time to start the quest.

Keldorn gets on with good and neutral companions, although as a paladin his definition of right and wrong is somewhat black and white. He will leave the party permanently if you choose any evil paths or if your reputation falls too low. He also has a side quest early on which involves his family and results in him leaving the party temporarily. Depending on how you resolve this he will either rejoin the party or leave permanently.