Ben Tennyson and Kevin Levin have one of the closest relationships in the franchise.
History
Ben 10
In the original series, Ben met Kevin when the game he was trying to play "ate" his money. From there, Kevin appeared and used his powers to give Ben some tokens. Despite a warning from Gwen, Ben runs away with Kevin into the streets of New York City. After nearly getting caught by police for trying to play a Sumo Slammers video game before it came out, the two form a "partnership" and proceeded to rob a train full of money. Ben finds out that the train had people in it and decides to stop Kevin from killing innocent lives, causing a fight between Heatblast and a Pyronite-mutated Kevin. Ben successfully stops the collision of the train, but Kevin escapes and goes loose on the city. After being picked up by Max, the trio go to where Ben expects Kevin to be and soon a fight between Four Arms and a Tetramand-mutated Kevin begins. After losing, Kevin fakes wanting to join Ben and the others in order to try to steal the Omnitrix, only to seemingly fail. It's then revealed that Kevin had actually absorbed the alien's abilities. Throughout the rest of the series, Kevin was constantly tormenting the trio and tried to kill Ben every week as stated by Ben in Perplexahedron.
Alien Force
Ben doesn't trust Kevin at first, but he then learns to trust him enough to allow him to join his team which happened because of the events of a future episode. During this series, they shared a friendly rivalry. In Ben 10 Returns, Ben and Gwen find Kevin making an illegal deal with other aliens. After a fight in which the Omnitrix changes, Ben and Gwen ask for Kevin's help in taking down the group of aliens. At the end of the episode, Kevin agrees to join Ben's team. Their relationship continued to grow as the first season progressed, as Ben helped Kevin get ready to go to Gwen's school dance.
Ultimate Alien
Their bond has become more stable than before, but in Absolute Power: Part 1, Ben sees that he needs to kill Kevin in order to prevent him from hurting more people. This caused Ben to become a ruthless person despite the fact that his facial expression changes to a positive one when Darkstar reveals that there's a way to cure Kevin. In the end, the two go out for smoothies after their short quarrel. In The Ultimate Sacrifice, Kevin admitted that Ben is truly his best friend.
Omniverse
They are still shown to be best friends whenever he and/or Gwen comes to visit though he is annoyed when Ben unintentionally wrecks his car during their battles with villains. They also enjoy cracking jokes and laughing with each other as in Mystery, Incorporeal when they were disrespecting Professor Xagliv's lesson, where she then dismisses them along with Rook and Gwen who angrily snaps at them while they try to look innocent. In Weapon XI: Part 1 and Weapon XI: Part 2, their friendship is once again put to the test as the Rooters believed Ben to be a dangerous threat known as the "Coming Storm" since he has the Omnitrix. Kevin turns on Ben claiming, they're not friends and joins the Rooters and the Amalgam Kids to end Ben, but at the last minute Kevin reveals that he was faking the whole time and that his plan was to set himself and the Amalgam Kids free from the Rooters' influence, stating to them that Ben is truly his best friend and like a brother to him.