Medessa stands as a primary antagonist, a malevolent witch wielding dark magic to further her aims. She forges a significant alliance with the villainous King Lavic, supplying him with magical protection and support in his campaign to conquer Camelot and thwart Arthur's rightful claim to the throne. This partnership establishes her as Merlin's direct magical adversary, countering Arthur's mentor and protector.

Her diverse magical abilities frequently manifest destructively. She commands dangerous supernatural phenomena, conjuring entities like a life-force draining magical flower. She employs illusions and phantom projections, notably crafting a deceptive phantom of Princess Ginebia to sow confusion or gain strategic advantage. Furthermore, she controls powerful creatures, including a fire-spurting dragon, deploying them as instruments of terror or direct assault against Arthur and his allies.

Medessa actively participates in the conflict, orchestrating schemes to hinder Arthur's progress and secure Lavic's reign. Her actions drive episodic conflicts, leveraging her magic to challenge the protagonists. Her ultimate goal aligns with maintaining Lavic's power and preventing Camelot's restoration under Arthur. Her defeat becomes a crucial objective for Arthur and his knights, underscored by the episode title "Defeat the witch Medessa!", signifying her pivotal role in the central struggle against Lavic's tyranny.