The good professor, to which all the PCs have some relation.
This is the character that draws the entire party together. Everyone’s character has some tenuous connection to him.
He is, in short, your stereotypical grandfather figure.
How did your character meet him? What were your character’s interactions with him? What caused him to grow so attached to you to seek you out out of the blue? These are the things that you should be thinking about as you develop your character story.