Elena Validus

Elena Validus is 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: 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 betray 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 evolved into her true Nanochip Queen 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 False Queen 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.

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.

Elena is also able to turn into a human-size Human/Nanochip hybrid form, and 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.

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, possibly due to her jealousy of humans of alien descent and their special powers (like Gwen and Kevin). 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.
 * A plot hole in Alien Swarm was that, during the Humungousaur scene, she knew Humungousaur's name, despite Ben not saying it when he transformed.
 * Elena's Nanochip form resembles a mix between Inque of Batman Beyond, Venom of Spiderman and Aku of Samurai Jack.
 * Because Elena changes shape in Revenge of the Swarm with the influence of nanochips, mutating almost is similar to how humans become E.V.Os in Generator Rex.