The Hero of Stealer of Souls was a "hardy and experienced adventurer" who was recruited by the wizard Vanestin in Pollua to rescue the wizard Alsander from the Isle of Despair where he had been imprisoned by the evil Mordraneth.
The hero succeeded in this task, only to discover from Alsander that Mordraneth was also on the Isle of Despair. Alsander then taught the hero how to cast three spells, before teleporting back to Pollua. The hero then went on to defeat Mordraneth and escape the island.
It is possible that the hero also took part in other books, either before or afterwards. Although the hero learned some spells, it seems that he or she did not become a wizard (as each spell could only be cast once), but was rather lent a part of Alsander's magical powers, so this should not unduly affect continuity.