Rupert is a powerful Mage seen in many adventures. He rarely shows hostility towards the adventurers, but always plays by his own rules. Like most immortal Mages, he owns a magical collection : he likes to steal the heads of headless knights and lure their bodies into his deadly dungeon.
Basing his magic on wizardry like all great Mages, he would lose his powers with his virginity.
Rupert is introduced as a perverted mage who enjoys lining up heads in his dungeon, including the hero's, Headless Knight, who attempts to rescue its own head.
He was victorious, even though he did not have to fight the corpse himself, and holds the head ever since.
Rupert never met the thief in his original form, believing him to be a very young and talented thief girl. He helped him out of his dungeon on the promise he'd stop trying to steal.
He later traded with her a precious item for immense amounts of money, hinting that he might have a contact with her in some way.
When she was young, the Aetheromancer went to get her grandfather's village back in her family's name. The village is on the lands below the Mage's fortress, and when he learned about her precious staff artifact, he decided to take her cursed friend hostage until she gave her the staff - he would then uncurse her. She refused and fled, looking for another way to uncurse her friend.
But the Nomad Thief stole her staff. Pretending to know nothing about it, Rupert then forced the Aetheromancer to be at his service until she found it back - she thus served many years as his minion. Ultimately, it seems he freed the Aetheromancer's friend Marzi and gave her the staff back, though the conditions of the deal are unknown.