Ben Tennyson and Julie Yamamoto

Ben Tennyson and Julie Yamamoto was one of the main couples alongside Gwen Tennyson and Kevin Levin.

History
Julie first met Ben as a crowd attendant in Ben's soccer game. She congratulates Ben on his win and tells him she has heard a lot about him.

Alien Force
In Pier Pressure, Ben hesitantly first asks her out on a date, much scared about Julie's reaction when she found out about the Omnitrix. Their date is interrupted when an alien named Ship kidnaps Julie. During the rescue attempt, Julie finds out about the Omnitrix and finds it cool instead of running away like Ben thought. They then decide to keep Ship and went home holding each other's hand.

In Good Copy, Bad Copy, she seems to be tutoring Ben in Physics, when they are going to have a test. She seems to be appreciating him that he was getting better day by day just to make him feel better.

In Save the Last Dance, Julie and Ben, both seem to accept truly that they share the "boyfriend-girlfriend" title. Julie tries to help Ben by following his alien form wherever it went, just to make Ben feel better. She also openly accepted that if there was any problem in Ben's life, she would be always there for him. At the end of the episode, when Kevin teases Ben of being a "mother", Julie unsarcastically tells Ben that he made a great mommy. In this episode, the actual lovestory of Ben and Julie starts.

In Pet Project, Julie seemed to have felt Ben to be very honest, polite, kind and cute. Later, she started to get mad with Ben because she thought he didn't trust her. Although at the end it seemed that Julie took her worthy time revenge on Ben as Ship fell on Ben getting him afraid and hitting him. In this episode we got to see Ben get extremely protective about Julie and bearing no nonsense regarding her, and would try to do anything that she likes. This episode also seemed to be the first open argument of Ben and Julie.

In War of the Worlds: Part 1, Ben seemed to be a little worried about Julie, as she did not have powers of her own and did not want anything to happen to her. Julie also shows a great concern of deal and affection for Ben. For instance, when Ben goes to the Highbreed home planet, Julie said: Be careful, Ben.

At the end of War of the Worlds: Part 2, Ben and Julie get close to kissing, but decided against it, as Gwen, Kevin and Grandpa Max were all looking on. Instead, Ben promises to walk Julie to the school and she gives him a loving smile and gets home by riding Ship.

In Vreedle, Vreedle, when Julie is being troubled by the Vreedle Brothers, Ben comes to her rescue and chases the Vreedle Brothers away. Then Ben and Kevin travelled half way across the galaxy only for Julie. This shows that Ben extremely loves her and cares about her. At the end, Julie too, said that she was very concerned about him.

In Single-Handed, Ben's cut-off hand finds Julie for help, rather than going to Gwen and Kevin. This shows that Ben trusted Julie the most. Also when Gwen told that she could not locate Ben through her powers, Julie was almost broken down until Kevin told her that he was alright. When Ben came back to the Earth, Julie was very happy to see him alive and gives him a tight hug. She even tells that she would catch Ben's hand as a weapon for the situation (although Gwen found it romantic as she thought Julie meant marriage).

Ultimate Alien
The couple first appears in Fame, when first Ben goes to Julie's house in order to avoid the media and then takes Julie on a long drive in Ben's new car, the DX Mark 10. At the end of the episode Ben and Julie share their first kiss (although Julie kissed Ben on his cheek). The kiss also gave Ben a lot of confidence and he was utterly shocked at Julie kissing him. This episode also proved that the couple were at a peak in their relationship.

In Duped, Ben makes three of himself so he can do whatever he wants. But Julie found out, and she was upset because the original Ben chose to go to the Sumo Slammers movie, leaving the immature Ben to watch Julie's match, while embarrassing her in the process. In this episode, Ben and Julie both admit publicly that they are a couple. After Julie found out what Ben did she got mad and stormed off.

They reappeared in the episode Hero Time, where at the beginning Julie tries to take up for Ben that there is nothing between him and Jennifer Nocturne. She is no longer angry at Ben, but later she gets jealous of Jennifer and removes her anger out on the tennis balls when Overlord kidnaps her. Later, everything goes well with Ben and Julie again after Ben goes Goop, saves Julie and calls her "his girl". It is now confirmed that they are back together again, and their relationship is still strong.

In The Transmogrification of Eunice, despite the events of Hero Time, Ben believes that he and Julie have broken up again (though Gwen indicates that Julie only said they might as well be, so it's confirmed that Ben is mistaken) and shows an obvious attraction to Eunice (a mysterious girl they discover in a crashed escape pod, who unknown to her or Ben is actually the human form of the Unitrix) and at one point he almost kisses her (only for them to be interrupted by Sunder). Eventually Ben discovers Eunice is in reality an early prototype of the Omnitrix created by Azmuth. It is also revealed by Azmuth that Eunice possess a randomization of Gwen's DNA (sampled by the Unitrix when Gwen touched its escape pod), which technically makes her a genetic relative of Ben (not to mention Gwen and a relative of the Tennyson family). Though Ben may still have feelings for Eunice, it is completely likely Ben's brief romance with Eunice won't continue due to her leaving to assist Azmuth on Primus and the circumstances of her origin.

In Eye of the Beholder, Ben and Julie were going through relationship problems. By the end of the episode, Ben apologized to Julie for being such a jerk and he promised to try to be a better boyfriend, which makes her much more happy, therefore resolving their relationship troubles.

In Revenge of the Swarm, Julie was kidnapped by Elena Validus in order to keep Ben from her. Later, when Ben saves Julie, she convinces Elena to stop her nanochip activity if she really loves Ben. After Elena kills herself to save Ben's life, Julie comforts him before they drive home. They are still dating.

In Prisoner Number 775 is Missing, Ben asks the police if his daughter is cute (when he told them that, his daughter would not stop talking about Ben). This hints that Ben is just joking about it, but Ben himself and Julie are still dating.

In The Perfect Girlfriend, their relationship spikes once again when Ben decides to chase after Ssserpent as Julie leaves for a tennis tournament. During the entire episode, Elena Validus makes a comeback disguised as Julie, asking what Ben would want. Her identity is not revealed until the end of the episode, where Ben realizes that Julie had left for her tournament. However, Julie returns to see Ben, but Elena makes her attack. As her nanochips choke Ben, Julie tells Elena to stop, otherwise if she kills him, then no one could have him. Elena retreats after this, saying she now "knows what hate feels like."

Inspector Number 13 shows Julie and Ben playing a tennis match. When Techadon Weapon Master Number 13 kidnaps Ben, Julie beats him with her tennis racket, proving that Julie really loves Ben by risking her life for him.

In The Ultimate Enemy: Part 2, Julie talks Ben out of selfishly wiping out all evil in the universe. After he comes back to his senses and returns all humans back to normal, she kisses him on the lips.

Omniverse
They've officially broken up, and are no longer a couple as in Rules Of Engagement. However, they agreed to be friends.

Trivia

 * Ben and Julie's relationship parallels the relationship between Azmuth and Zennith, to the point where they share the same reason for the breakup: Ben and Azmuth simply didn't know that they lost their girlfriends until it was too late to get them back. Unlike Zennith and Azmuth, the former couple agreed to a friendship.
 * Derrick J. Wyatt confirmed they wanted to do something different for Ben and Julie as some of the producers didn't care about their relationship in Alien Force/Ultimate Alien.