Joshua, is an acquaintance of Rohen Tahir and friend of Tahira.
Before the Convocation
While the exact time isn't known, Joshua, together with Caele and (most likely) Lev was part of a group hunting the giant named Lordir (originally "Lördir"), lead by a Dwarf named Skjalf.
These events are depicted in the short story Guardian in the Mountain.
Battle for Fiara
He was a member of a group tasked by Rohen with retrieving a special rune from the ruins of Ankbrand which was in possession of an Orc chieftain named Krong. The group was ambushed by the Orcs and forced to abandon the mission, deciding instead to seek out their friend and Rune Warrior Tahira who lives nearby.
While Caele and Lev hurried north to Tahira's home, Joshua stayed behind close to the Rune Monument in the middle of the island waiting for them. After Tahira acquired the rune of the Phoenix Bearer, she gave it to him, who in turn gave it to Rohen Tahir.
His current whereabouts are unknown.