Zekar is a bandit in the Fisherman's Lake area northwest of Nashkel . Along with Jemby, he is part of Teyngan's band that will try to rob you and your party. Using just a Composite Longbow and normal Arrows, he appears able to inflict a potent poison with every successful hit, so he takes precedent over Teyngan in the kill order.
Note: He does not necessarily drop a Studded Necklace with Zios Gems. In that slot he drops one random treasure. Other possiblities include 57 gp.