What are your favourite background OCs

In my current fic I am really enjoying making up random OCs with no plot relevance that are only mentioned once.

Examples: R Lagu, a headmaster of Hogwarts from medieval times who due to nearly all of the records of their existence being lost is kindof mysterious. The only things present day people know about them is 1) they were headmaster of Hogwarts 2) they wrote a book on herbology and 3) they drowned in the hogwarts lake. Nothing else is known about them, "R Lagu" isn't even their real name, and people don't know their gender, when/where they were born or their house.

Other example. Matilda Mumbleberry, who wrote a book proporting to be about an aspect of magical theory but is actually memoir detailing the affair she had with the muggle sister of her best friend.

What are your favourite plot irrelevant OCs?