Grandpa Grumble is one of the main characters in the book Moominvalley in November. He claimed to be over 150 years old. He has problems with memory, and a host of medicines that he's supposed to take, but often forgets. He takes pride that his hearing is still intact, and it annoys him when people assume otherwise.
In the beginning of the story, he forgets his name, and in lieu of remembering it, he decides that Grandpa Grumble would be a better name for him. He is disillusioned with old age, seeing parties through people's windows which he wasn't invited to, and decides to walk to Moominvalley to try to engage with his youth. He has vague memories of catching fish in the stream that runs through it.