Branwen
Branwen is a True Neutral Cleric who can be recruited in BG1.
Baldur's Gate 1
Branwen can be found in the Carnival area (east of Nashkel). She has been turned to stone and you will need to use a scroll of Stone to Flesh (which can be purchased from most temples) in order to restore her. Once restored, she will offer to join the party. She has no joining conditions or side quests, and will remain in your party until dismissed.
As a cleric, Branwen is an essential addition to most good/neutral parties, unless the main character is also a cleric or druid. She can also hold her own in melee combat, although she is best used as a second-line fighter who can also cast defensive and healing spells.
Siege of Dragonspear
If present, Branwen will leave your party at the end of the introduction to Siege of Dragonspear and cannot be recruited again.
Baldur's Gate 2
Branwen does not appear in Baldur's Gate 2.