Julie Yamamoto

Julie Yamamoto is primarily Ben's love interest, and later becomes his girlfriend. She wears a black t-shirt mostly hidden under a pink sweatshirt, and a white skirt. When playing tennis she is shown wearing wrist bands, and a white skirt.

Alien Force
She first appears in Ben 10 Returns: Part 1, where she congratulates Ben after his team won a soccer game.

In Pier Pressure, Ben asked her out on a date to the pier. It was interrupted by an alien named Ship, who kidnapped Julie. During the rescue attempt, Julie found out about the Omnitrix, which she finds fascinating. In the end, it was revealed that Ship was trying to lead Ben to his "creator" named Baz-L, who's ship crashed and was about to explode. Ben and Julie helped fix Baz-L's ship and he left Ship (the alien) with them, saying that he and Ben have a bond. On the walk home, Ship takes off after a passing truck, and Ben and Julie walk home, holding hands.

In Save the Last Dance, when Big Chill takes control of Ben's personality and starts trying to eat every metal object in sight, Julie follows him on motor scooter and alerts Gwen and Kevin (who were on their way to Gwen's school dance) on her cellphone. She tells them where Ben is headed. Gwen and Kevin manage to subdue Big Chill, only to discover that Big Chill was only building a nest for eggs he had laid. The eggs hatch into infant Necrofriggians (Big Chill's species), and Kevin teases Ben about being a "mother". As Julie takes Ben home on her motor scooter, she unsarcastically tells Ben that she thought he made a great "Mom". Ben also stated she gets straight A's. In Pet Project it was revealed that she is keeping Ship as a pet. She also plays tennis as a hobby, as seen in Pier Pressure.

In War of the Worlds, Julie helped Ben fight against the Highbreed invasion. It turned out that she apparently had a curfew. She also seems to show a deal of concern and care for Ben. In Save The Last Dance, she hugged him after he went missing. In the end of War of the Worlds, Ben and Julie get close to kissing, but decide against it, as Gwen, Kevin, and Max were all looking on. Ben then tells her that he will walk her to school the next day, and Julie gets home by riding Ship. In Vreedle, Vreedle, two alien henchmen, the Vreedle Brothers, are hired by Baz-L (Ship's original master) to get Ship and bring him back. As Julie refused, the Vreedle Brothers started being more aggressive, and Ben and Kevin go in martial court to claimed their right on Ship against Baz-L, while Gwen stayed on Earth to keep Julie and Ship safe. When the Vreedle Brothers came back, Ship revealed he has the ability to "merge" with Julie, giving her an powerful robotic battle suit she can use to fight them. Ben is eventually able to get the judge's favor, and Baz-L reluctantly accept Ship is now Julie's.

In Single-Handed, Julie "finds" Ben's hand and takes it to Gwen and Kevin. She later uses the serve as Diamondhead to fight Sunder. During the fight Gwen helped her keep Ben's hand away from Sunder but wound up being used in a trade when Sunder caught her. After Sunder was knocked out she under Kevin's instructions held a beam on Ben's arm while they used Sunder's axe to put Ben back in one piece.

In the The Final Battle: Part 1 she is briefly seen playing with Ship when Gwen calls Ben because Kevin hadn't shown up.

Ultimate Alien
Julie, now 16 years old, will be again helping Ben's team from time to time. In the first episode of the series, Fame, she comforted Ben when his identity was revealed to the world. When he thought everybody hated him, she kissed him on the cheek (their first on-screen kiss) and told him that not everybody hated him. In the second episode, Duped, of Ben 10: Ultimate Alien, Julie became very upset and angry with Ben for not spending time with her but that's sure that Ben will make her up soon.

Abilities
Being an ordinary human girl, Julie doesn't possess any special powers herself, although she's quite good at tennis, and shows some fighting ability in Pet Project, (comparing her weapon to a tennis racket.) However, she's the only one able to fully control Ship, making her a competent ally.

In Vreedle, Vreedle, her pet Galvanic Mechamorph, Ship, reveals that it can 'merge' with Julie. This was seen so-far only in the episode Vreedle, Vreedle when Ship enveloped her, becoming a exoskeletal mecha battlesuit: In this suit, she demonstrates superhuman strength and durability, as well as the power and ability to fire various powerful weapons. It also seems that Ship would be the one controlling the armor as Julie commands it.

Trivia

 * Julie has a Japanese heritage, as her last name is "Yamamoto ," which is a common surname in Japan.
 * Julie owns a motorized scooter.
 * Her galvanic suit has a strong resemblance to the Samus Aran's Chozo Powersuit even to the point of having similar weapons. Exactly how this transformation is possible is as yet unknown, as it was previously stated that Mechomorphs cannot combine with living organisms.
 * Many fans were disappointed that Ben and Julie did not kiss in Alien Force.
 * Ben and Julie shared their first on-screen kiss (though only on the cheek) in the first episode of Ultimate Alien, Fame. Also, confirmed by Dwayne McDuffie, they will be sharing a lot more kisses in the further episodes.
 * Like Ben, Julie too will be not having any change of clothes in the Ultimate Alien series.
 * Gwen considers that Julie is the only one who has common sense among the four of them, as she always avoids getting in trouble.
 * Ben considers Julie in many episodes as his "voice of reason". It is also shown that no one can comfort Ben better than Julie.