Kivan is available from Chapter 1. He can be found in the High Hedge area at co-ordinates 3650, 1450. As befits a ranger, he blends in well with the forest environment. You can quite easily walk right past him on the path in that area without noticing him being there.
Kivan is a good archer, not as good as Coran, but close to Khalid. His high strength score lets him equip composite longbows by default, it can also be put into good use by making him a melee candidate as the damage output of most range weapons don't scale with strength except slings, throwing daggers or axes.
He starts with Stealth at 41, the special ability Charm Animal (x1), his racial enemy is ogre.
Kivan reveals that he hails from the forest of Shilmista, though he has not visited his homeland for quite some time. He has instead been traveling the world driven by the darkest of obsessions: revenge. Kivan and his mate Deheriana were apparently on a romantic sojourn years ago when they were waylaid by a group of Brigands. They were captured and tortured for the amusement of Tazok, the half-ogre that led the attack. Kivan eventually escaped, but Deheriana did not survive the encounter. Kivan is a man of few words, and obviously does not make friends easily. Likely, he will not take much pleasure from life until he has revenged the death of his love.
There are character files for Kivan in Baldur's Gate II: Shadows of Amn and he even has a character portrait (something the BG1 cameos lack), both indicating he may have been meant to be playable. [confirmation required]