Maxwell "Max" Tennyson, better known as to everyone in Grandpa Max, is the grandfather of both Ben Tennyson and Gwen Tennyson and took them on a cross country trip during the first series of Ben 10. He loves his grandchildren with all his heart and always makes sure to keep them happy. He is also a secret member of the intergalactic police organization called the Plumbers.
FusionFallTo help fight the Fusion invasion, Max is lending all of his Plumber knowledge to aid Dexter and Professor Utonium in learning more about Planet Fusion and its inhabitants. Thus Max is currently working on a satellite to survey and spy on Planet Fusion. Fuse is all too aware of this plot, and constantly sends his forces to sabotage Max's project.
While retrieving a battery needed for the satellite, Max not only finds the negative battery missing, but he also finds out that Ben's old evil alien form, Ghostfreak, and his people, the Ectonurites, are involved with Fuse after he spared their planet, Anur Phaetos, a long time ago. In return for his mercy, the Ectonurites have promised to aid Fuse in his plot to invade Earth. To defend Earth from this possible secondary invasion, Max has set up a field that will alert him if an Ectonurite does decide to land on Earth.
Grandpa Max is stationed alongside Numbuh One and The Scotsman in the ruins of Townsville Park and can also be located in ship's interior giving the Rath Nano mission, "Let Me Tell You About Rath!" He studies the giant Fissure torn in the middle of the park which is the area's Infected Zone. He suspects that Fuse is trying to awaken an ancient beast lying deep inside the Fissure. Many of Max's missions involve traveling into the Infected Zone (The Fissure) to impede Fuse's efforts. It was mentioned he is good friends with Plumber Nathan and Plumber Carol.