In Armic's scenario, his gang entered the Abandoned Castle in Zomar searching for some Obsidian Boots, but Vearst got scared and ran away.
In Ventus' scenario, Ventus and Tiffon asked for Vearst's help because they needed someone intimidating to get the carrier mission. Vearst doesn't have much beyond looks; he fell asleep and his previous party left without him. When he joins Ventus in a quest he manages to lose the Carrier Quest to monsters who take it to Dagul Bos' Castle.
Vearst's only really viable option in combat is to tank. He has poor stats mostly, except for an avverage Magic growth, good Water growth and high LP. By focusing on the Water element, with Support equipment and a few key spells, Vearst can be a very reliable and helpful character.