If I remember correctLy:
He was a close friend of Melinda and Merlin before they were famous. He had an unreturned crush on Melinda.
As the Sage and Guru grew in fame for their achievements, their friend was left behind. Despite his own impressive talent, he was constantly overshadowed by Merlin.
Eventually, resentment and jealousy got to him, and he SNAPPED. Merlin and Melinda were completely unaware of his growing resentment and were forced to ultimately put down their friend. The magic institutions covered up the incident by labeling him as a “devil” and labeling them as “devil slayers” or “heroes”. The guilt of being celebrated for having killed their former friend strained their relationship, and this is why they’re not currently “together” (married).