Cool grandfather

The cool grandfather is an unnamed "cool" grandfather who is the father of Mav. He wears a "promise stone" around his neck.

History
When Homer mentioned that Mav and Portia's house had everything, including a "cool" grandfather, he told Homer that he "loved two things: shutting up and giving away money" before giving Homer money. When Homer received the money, he said "thanks, dad!" whilst wiping away a tear from his eye. Soon afterwards, Lisa asked what that stone around his neck was and the "cool" grandfather told her that it was a "promise stone" and that it signified that if he was ever a burden, he'd jump into a volcano.