Grandpa John is a character in the twenty-second Goosebumps Series 2000 book, Full Moon Fever. He is the grandfather of Robbie and Aleisha.
Siblings Robbie and Alesha go to their Grandpa John's house. When they are there, John tells them a scary Halloween story about a curse that if you look at the full moon, you'd become a snarling, ravenous beast. He manages to scare the two, but he pretends to have caught it as a joke, so they shrug it off.
On Halloween night, he lets Robbie and Alesha go trick-or-treating through the neighborhood, completely unaware of them going to Mrs. Eakins' house. The next day, though, the two come to him looking like the snarling, ravenous beasts he talked about in his story. When Robbie and Alesha ask him about Full Moon Fever, he tells them that it is merely an old legend, therefore it isn't real.
The two kids then escape to Canada, looking for the woman who told John the story. John is unaware that an evil doctor has captured the two and has turned them into circus freaks. After a few weeks, the two return to Grandpa John, who is nervous. This is his last appearance in the story, as Robbie and Alesha are at Mrs. Eakins' house for the rest of the story.
He has pale skin, with white hair, and bulging watery eyes.
John comes off as a fun, spooky, loving grandfather.
He is easy-going and strong, but he also has a nervous and weak side, as shown when Robbie and Alesha return home from captivity in Canada. He loves his grandchildren, but he will have fun by telling them a good tale. He is brave in front of the two beasts/children, and doesn't care about werewolves or beasts.