Bivalvan

Bivalvan (a play on bivalve, a term for clams and oysters) is a member of a species from a thus-far unknown planet in the Andromeda Galaxy.

History
He was first seen in the Ben 10: Ultimate Alien premiere. Every night he would rise out of the ocean and raid NASA, stealing equipment from their secret rocket project. Ben, Gwen, and Kevin attempted to stop his latest raid, but Ben's Ultimatrix detected that his DNA was uncataloged in Primus' Codon Stream and went into sample mode, leaving Ben unable to transform while Kevin and Gwen fought him and were beaten. Ben attempted to fight him without the Ultimatrix, but it didn't work well. Finally, the Ultimatrix finished obtaining a sample of his DNA and allowed Ben to transform into Chromastone, who unfortunately, was defeated just as quickly. It was a revealed this time that he took the rocket's engine, which was a nuclear bomb. Tracking him to his underwater cavern hideout, Ben and the others realized he was stealing the rocket parts to build himself a spaceship to return to his home planet in the Andromeda Galaxy. However, since he was going to use the nuclear bomb to trigger his launch, Ben and the others had no choice but to stop him. With Gwen and Kevin trapped by Bivalvan's welding torch, Ben had to fight him on his own as Spidermonkey. However, Bivalvan still proved too much for him so he transformed Ultimate Spidermonkey who quickly defeated him.

Bivalvan then revealed that he and four others (possibly NRG, Armodrillo, Terraspin, and Ampfibian's species, judging as how Water Hazard was listed as a new alien in Ultimate Alien, but was absorbed into the Ultimatrix, the others listed an new aliens may be to), were kidnapped by a monster called Agreggor before they escaped to Earth. Ben and the others left him behind while they took the bomb back to NASA, saying they would contact the Plumbers to ensure Bivalvan would get home. However, before the Plumbers arrived, Agreggor appeared, saying that no one escapes him. Bivalvan was then heard screaming offscreen, indicating he was either killed or tortured before being recaptured. If you listen closely in the background when Agreggor walks towards him off-screen, you can hear a ripping sound as Bivalvan is screaming indicating he screamed because Aggregor ripped off the webbing and proceeded to recapture him. Bivalvan reappears in Escape From Aggregor in Galapagus's flashback with lighter voice alongside Galapagus, P'andor, Radd, and Andreas. Bivalvan seems not violent but a leader and he dislikes P'andor. It is revealed that Agreggor intends to absorb Bivalvan's powers along with the powers of the others, meaning that Bivalvan is probably still alive.