Alien X is a Celestialsapien ("celestial" referring to a supernatural being and "sapien" meaning "wise" in Latin) from the planet Zvezda (meaning "star" in the Slovenian language, or "knowledge" from veda in Sanskrit). Alien X is capable of warping reality at a whim. He has a black body covered with tiny white stars, black feet, green eyes, and three horns protruding from his head. Alien X possesses three distinct personalities: Serena (voiced by Vicki Lewis), the voice of love and compassion; Bellicus (voiced by Kevin Conroy in the series and Jeff Bennett in the Ben 10 Alien Force : Vilgax Attacks), the voice of rage and aggression; and Ben, the voice of reason. All three speak in unison when Alien X speaks.
In order for Alien X to perform any action, including but not limited to simple movement and transforming back into Ben, two thirds of Alien X's personalities must agree to do so. As both Serena and Bellicus have been arguing for an eternity, and before Ben had no tiebreaker, getting them to focus on any present task is difficult at best, leaving the body of Alien X frozen in place while Ben attempts to convince them to act. Alien X first appears in "X = Ben + 2", where Ben uses the form to stop a flood caused by a broken dam. After much fruitless argument, Ben eventually convinces them to let him leave and use a different alien form to stop the Incursions. After the threat is over, Ben resolves to never transform into Alien X again, stating that it isn't worth the price the power comes at.
Alien X appears as an NPC in Ben 10 Alien Force : Vilgax Attacks. When Psyphon starts to use the Null Void projector on Ben, Paradox tells Ben that now is the time to use Alien X. Upon transforming Alien X, Ben tells Bellicus and Serena that if they don't help him save Earth, it will be destroyed along with them. This causes Alien X to use its powers to deflect the Null Void Projector's beam right back at Vilgax and Psyphon.
Alien X appears again in the "Ultimate Alien" episode "The Forge of Creation," where Ben turns into him to try to find the Forge of Creation by trying to get them to cooperate. But when that fails, Paradox shows up and transforms Ben back into human.
Apparently Bellicus and Serena both hate Paradox and voted with each other to destroy him, before he turned Ben back to human.