Waver Velvet stole his teacher’s artifact and joined the Fourth Holy Grail War in the events of Fate/Zero. Despite having a strong bond with Rider, he ultimately lost the war. He then returns to the Clock Tower for further studies, and became a professor. Since his teacher died in the war, he also inherited the title of Lord El-Melloi, after convincing from the next heir.

Correction

Um actually … Waver did not inherit the title of Lord El-Melloi. He insisted that he is titled Lord El-Melloi II, since he felt unworthy to be compared to his teacher.