Lannie Caplain was the brother to Lahnik "Lahni" Caplain. He went missing after a rival family, the Coymacs, poisoned the Caplains' possum cake, and he was blamed for it1. Lahni has been searching for him since, with the help of The Tower of Grey (though recently, they seem to have postponed that particular quest).
We saw, many years ago, Lannie encounter a young Skud Derringer at The Monastery of Uthgar, when The Skull Bandits attacked said Monastery. Lannie seemed to have been a member of the Bandits. He claimed that his "peepaw" had told him that this Monastery contained untold riches, resulting in his interest in the place. It is assumed that he told the Bandits what his "peepaw" told him. Lannie was timid, and didn't hurt anyone during the attack. As a result, he was sparred when Master Alder entered the Monastery and killed the Bandits. Alder took Lannie to "The Constable" for questioning. As of now, the name, location, and status of this "constable" is unknown. Lannie's status, is also unknown.