Elena Validus

Elena Validus is a Plumber's kid who served both as an ally and enemy to the heroes in the series, as well as one of Ben Tennyson's love interest. She was first introduced in the live Ben 10: Alien Swarm. In Ultimate Alien, she has medium length black hair, black eyes with long lashes(possibly mascara). She wears a white lab coat over a v-neck black mini dress and black flats while in Alien Swarm, she wore a red leather jacket over a black shirt, dark blue leather pants and black heels. When she was out with Ben, Gwen and Kevin, she had red lips.

Ben 10: Alien Swarm
According to the movie, Elena used to hang out with Ben and Gwen. During this time she became close to Ben, he helped her join the school soccer team, speaking up for her when the other players wouldn't let her join. She eventually became the team's most valuable player in the live-action film. Gwen told Kevin that Elena was Ben's first major crush, but she was forced to leave Bellwood after her father was caught stealing alien tech from the Plumbers (Max never told Ben or Gwen why Elena and her father moved away).

She eventually discovers her father had been taken over by the Hive Queen, and seeks help from Ben and the Plumbers. She sets up an illegal alien tech deal, in the hope that the Plumbers would catch her so she could warn them about the alien chips. An undercover Kevin acts as the buyer along with Gwen and Ben. Elena reveals herself to Ben and Gwen, who are stunned by her sudden reappearence. They are then attacked by Victor Validus (possessed by the Hive Queen). Ben defeats the alien chips as Big Chill and Validus escapes. Afterwards Ben, Gwen and Kevin talk about what had just happened. Though Ben trusts Elena, Gwen and Kevin don't (believeing she set them up). Elena secretly follows them to Max's Plumber HQ. At Max's Headquarters, she is detected sneaking inside of Max's office and is caught. She tries to explain herself, but Max tells Ben and the others that they can't trust her. Her plea for help rejected, she is lead out by Kevin. Riding her motorbike, she is stopped by Ben (riding Max's motorcycle) who decides to help her find her father, against Max's orders. They head to her father's secret lab to search for clues, but are attacked by humans enslaved by the Hive. They barely escape with their lives, as Elena crashes her bike, and is forced to leave it. She climbs onto Max's bike with Ben and the two of them flee. They make their way to the warehouse of a shipping company (called "Ship-It") her father was using, when they encounter Kevin and Gwen being chased by swarm of alien chips. Ben saves Gwen and Kevin (destorying Kevin's car in the process).

Elena shows them some of the alien chips they found at her father's lab, and they all head back to HQ to study them. They are confronted by Max who is angered by Elena's presence. Ben, Gwen and Kevin show him the active chips, and realizing Validus may have been right all along allows Elena to help them. Ben later discovers one of the chips is missing and is attacked by an enslaved Max. Max is subdued by Kevin but later escapes. Elena and the others calculate where the alien chips are being produced; Ship-Its main destribution hub. Elena, Ben, Gwen and Kevin sneak into the main warehouse, where they discover Validus hooked up to a machine. They see Validus producing the chips from his body, revealing to everyone that he is the Hive Queen. Gwen and Kevin confront Elena, who reveals she knew it all along, but didn't tell them because she was afraid she they wouldn't believe it (just like how the Plumbers didn't believe her father). Gwen and Kevin believe sacrificing Victor is the only way to stop the Hive and save the world. Elena begs Ben to save her father. Ben tells them he will use the Omnitrix to transform into a member of the Hive (having realized that the Omnitrix had been scanning the Hive chips earlier). He uses the Omnitrix to transform into the his new alien Nanomech. As Nanomech, he travels into Validus's body and defeats the Queen, freeing him from her control. Elena hugs Ben and thanks him for sparing her father's life. As they are leaving to return home, she says she plans on joining the Plumbers, but gets into an argument with Gwen.

Ben 10: Ultimate Alien
Elena makes her appearance in Ultimate Alien in the episode Revenge of the Swarm. After Ben was repeatedly attacked by nanochips in the form of Victor, he investigates the Validus Laboratories accompanied by Kevin and Gwen.

A skirmish with the janitor leads them to come to terms with Victor's death, though they are skeptical of the claim. They traveled to Plumber's Academy in hopes to find Elena, who knew most about the chips. Despite being one of the best students she dropped out of the academy in order to pursue her father's research.

The trio went to her laboratory and Ben was greeted warmly by Elena. She explains that the nanochip Ben saved wasn't hers and that she would not want to go over these matters as her father's research matters more. Kevin and Gwen were attacked by Victor's nanochips form and Ben comes to rescue them. The nanochips flee somewhat giving the expression of not wanting to fight Ben.

They went to the cemetery where Gwen confirms that Victor is truly dead. Elena surprises them with her choice wanting to help them over continuing her experiments. They went over to Mr. Smoothy where Ben mentions Julie which irked her. The nanochips then kidnapped Julie. Ben confronts Elena with the nanochip he found and berates her.

It is revealed that she lied and that her true intentions were to have Ben all to herself even if it meant harming everyone that stood in her way. She transformed into her Nanochip Queen form and prevented Ben from getting to a storage shed where she locked up Julie. She admitted that the Queen Ben killed in Alien Swarm was just a decoy and that the nanochips were residing in her since a long time ago making her their queen. Ben retaliates and frees Julie.

Elena generates nanochip clones of Victor to attack Gwen and Kevin after they found out that Elena is the Queen on their own prior to their arrival. She had Ben in a death grip within the nanochip's grasp. Julie reasons with her if she did ever truly cared for Ben she would stop this. Elena, still having feelings for Ben, agrees with her.

She sacrifices herself despite Julie's calls and was destroyed coming into contact with the nanochip force field she showed earlier. Julie tries to console Ben saying that the nanochips took over her while he said she was still his friend. At the end, several of the nanochips are still active.

Later in The Perfect Girlfriend, Elena was revealed to be alive despite her sacrifice but returns to her villainous ways when she disguises herself as Julie. In her Julie form, Elena tried to do everything possible to make Ben happy and allow him to get anything he wants.

She also injured Gwen's ankle by pushing her down an elevator shaft and attacked Kevin with a duo of building-monsters while he was spying on her but Ben defeated it with Way Big. Once Ben finds out that "Julie" is really Elena, he fights her as Ultimate Echo Echo but loses after she chokes him with nanochips.

Julie tells Elena that if she kills him, then no one can have him. At the end of the episode, Elena retreats into the sewer, "knowing what hate feels like" and promises to return.

Powers and Abilities
When first introduced, Elena was, unlike most Plumber's kids, depicted as 100% human and had no natural powers. However, she was mentioned to be a great athlete, Ben describing her as their school Soccer's team's MVP. She was also shown to be an exceptional martial artist during the fights against the nanochip's human hosts, which might be explained by her father providing her with basic Plumber training similar to Ben's. In Revenge of the Swarm, her former instructor at the Plumber Academy mentioned she would have become a great Plumber.

Since Elena was transformed into a Human/Nanochip hybrid form, she demonstrated the ability to control the Nanochips, using them to fulfill her deepest desire (in this case, her previously repressed crush for Ben). She could shape them at will, creating various structures such as copies of Ben's car or clones of her father she used as minions. She can also use them to animate buildings and control technology. Since her body is essentially made of Nanochips, she also has the ability to change her own appearance at will, and control her form to produce tendrils and melee weapons to use.

Trivia

 * In Alien Swarm neither Elena nor Ben admit they like each other, but in Revenge of the Swarm she wanted Ben all to herself. Though it should be noted Elena was under the Queen's influence at the time.
 * Elena appears to be slightly xenophobic. When introducing herself to Kevin she explained she was human and wasn't a freak like them (referring to Kevin and Gwen).
 * Elena appears to have something of a friendly rivalry with Gwen, similar to the rivalry between Ben and Kevin.
 * Elena's Nanochip form resembles a mix between Inque of Batman Beyond, Venom of Spiderman and Aku of Samurai Jack.