Magister Maxwell "Max" Tennyson or Grandpa Max as his grandchildren call him, is the paternal grandfather of Ben and Gwen. He is a fairly overweight man in his late 50's, with a keen sense for adventure and a strange taste in food. Max ends up taking the kids on a summer vacations' road trip across the United States. Max travels in a motor home nicknamed the Rust Bucket, which he has modified with advanced technology.
Max was a semi-retired legendary Magister rank Plumber and is the mentor of Ben, Gwen, Kevin, and the Plumbers' Helpers. Max is widely considered the greatest Plumber in the Milky Way. As of Omniverse, Max quit retirement and joined the Plumbers again, and is currently Magister of Earth.
Maxwell "Max" Tennyson is the 60-year-old paternal grandfather of Ben and Gwen. Despite his age, Max has shown on multiple occasions to be highly athletic and a formidable fighter. He takes his grandson gaining superpowers and super-abilities in stride and supports Ben in any way he can. Max was once in the United States Army, an astronaut program, and was a Plumber. He has been through his share of scrapes; Ben and Gwen did not know of this at first, believing him to have been a literal plumber. They began questioning this after several incidents in which he displayed knowledge and skills that a plumber would not normally have; everything from military-style hand signals to a working knowledge of nuclear reactors. Throughout the first season, it seems that Max knows far more about aliens than he lets on, which climaxes in the season finale, where it is revealed that he and Vilgax are old enemies, with Vilgax even going so far as to call Max "a thorn in his side". Before Ben got the watch, Max tried to keep the Plumbers and the aliens a secret from his family because he wanted to protect them and he did not want this life for his family; however, his plan failed when the Omnitrix and Vilgax came to Earth and he eventually accepted Ben and Gwen's involvement with this life. He begin to train them how to use their skills on the field. Keeping his secret from his sons had led his sons to resent him, and he never gave his sons a chance to learn about his life. He eventually learned the errors of his ways and he begins to realize that Ben and Gwen have what it takes to fight evil.
In reality, Max was indeed a plumber, but not the kind that Ben and Gwen thought he was. He was part of an organization called "The Plumbers", and Max is one of the leaders, known as a "Magister". It is unknown if Max's strange taste in food was acquired from his adventures to other planets during his career as a Plumber, or if its root can be found in his childhood. Max was to have been among the candidates eligible for the Apollo 11 mission, and would have gone were it not for other concerns.
He drives a recreational vehicle nicknamed the "Rust Bucket." Although it appears old and dilapidated, it conceals a wide range of weapons and technology issued to the Plumbers.
Max is an old friend of an alien named Xylene. She, like all the other three-eyed aliens of her planet, has the powers and abilities of flight, telepathy, and telekinesis. She had intended to send the Omnitrix to Max, but Ben found it accidentally during the camping trip. They seem to have been romantically involved and work well as a team. Xylene tries to convince Max to form a partnership and travel across the galaxy with her, but Max eventually turns her down twice: the first time due to his duties as a Plumber, and the second time due to his love for the kids.
He also had quite a genuine strong and close romantic relationship with an Anodite alien named Verdona. They ended up having two sons, Carl (Ben's father), and Frank (Gwen and Ken's father). However it seems that their mutually strong romance had slightly deteriorated as Verdona left Earth to return to her home world. She was assumed dead by her husband in Ben 10: Race Against Time.
In Ben 10: Alien Force, Max plays a part in exposing the Highbreed conspiracy. He destroyed a Xenocyte hatchery by detonating a Null Void Projector, which, after removing its focusing lens, makes a "pretty good imitation of a hand grenade" and produces an explosion that covers over half a mile wide. Max appeared to have been killed in the blast, but was instead sent to the Null Void where, calling himself "The Wrench", he fought against Doctor Animo (who had taken over the Null Void). Once the conflict was over, Max trained a new group of young Plumbers while at the same time informed Ben and the team of any alien threats.
In the original series, Max wears a white shirt, covered by a red button up Hawaiian shirt with a floral design. He has grey hair and blue pants, as well as black shoes.
After being turned into a ten year old in Don't Drink the Water, Max can be seen wearing Ben's regular clothes, a black-striped, white shirt with short sleeves, military green cargo pants and black and white sneakers with black stripes. His hair is also similar to a crew cut, and rather than fat, Max looks skinny and athletic.
As Ultra Grandpa in Perfect Day, he has a stereotypical superhero costume. He wears frameless blue goggles, a cyan shirt and white pants. He also dons a white cape like Ultra Ben.
In Alien Force and Ultimate Alien, Max's appearance did not change except for his blue pants being changed to brown, he has more wrinkles, a slightly darker shirt, and whiter hair.
In Alien Swarm, Max wears a blue Hawaiian shirt instead of a red one.
In Omniverse, Max's appearance has not changed that much except for his brown pants, which are now white. He has whiter hair, more wrinkles, and his eyes have been changed to little black dashes. His white shirt that he wore under his red button up shirt has changed to black. Also, his shoes are now grey at the top and black at the bottom.
In the reboot, Max has grey and white hair, and has brown eyes. He wears a white shirt with a red Hawaiian shirt with orange clovers on top, and has a brown jacket on top of it all. He wears blue jeans and brown shoes. He has a brown and gold watch on his right arm.
Secret of the Omnitrix
Race Against Time
Destroy All Aliens
Ben 10/Generator Rex: Heroes United