Tetrax Shard, dubbed "Hoverboard" in his first appearance for his namesake flying device, is a Petrosapien mercenary, hired by Vilgax in the episode "Hunted" to take the Omnitrix from Ben. He has nearly impenetrable gray armor, a black helmet with a small, green triangle (presumably for sight) on it, and carries a large rifle. His Petrosapien physiology can manifest through the armor, allowing him to use his powers without removing it.
Tetrax in his first appearance
Unlike the other hunters, Tetrax is allied against Vilgax. Upon first discovering that Ben has the Omnitrix, he laments the fact that out of the planet's six billion inhabitants it would end up on the wrist of a child. He also berates Ben's lack of strategy in regards to its use. After helping Ben to defeat the other hunters, during which Ben learns to use the Omnitrix strategically, he decides that the Omnitrix is in good hands, allowing it to remain with Ben. He also gives Ben his hoverboard, saying that it might come in handy. Ben uses it several times throughout the series. In Ben 10: Secret of the Omnitrix, the hoverboard is melted in a vat of acid, but Ben is given a replacement.
Secret Of The Omnitrix
Tetrax returns in Ben 10: Secret of the Omnitrix, where his past is explained. As a being who cared about no one but himself, he became a mercenary and ended up hiring himself out to Vilgax. When he finished the job Vilgax gave him, Vilgax created a weapon (using the crystal Tetrax stole) and used it to destroy the Petrosapien homeworld when they would not bow down to him. This caused Tetrax to betray his former employer, and he now works as the Omnitrix's Guardian and to rid the galaxy of his evil. He arrives on Earth after receiving the Omnitrix's SDM (Self Destruct Mode) signal, and takes Ben aboard his ship in search of Azmuth, the creator of the Omnitrix and the only one who can deactivate the self destruct function. Unfortunately, Tetrax's problems are multiplied when Gwen stows away aboard his ship, and Ben's transformations are revealed to accelerate the countdown. On the planet Incarcecon, Tetrax runs into Sixsix, another of the bounty hunters hired by Vilgax who ended up on the prison planet. After a brief battle, he and the Tennysons find Myaxx, Azmuth's former assistant and a DNA Force member like him. Because of her, Tetrax is forced to reveal that the Omnitrix's explosion will create a shock wave affecting the entire universe, instead of just the wearer as he told Ben. Tetrax then leads the group to Azmuth's compound on Xenon, where he faces Sixsix again before defeating him by delivering a fatal blown to his head. After Vilgax is defeated and Azmuth deactvates the self destruct, Tetrax returns Ben and Gwen to Earth and departs.
Alien Force
Tetrax plays a central role in the episode The Secret of Chromastone when he comes to earth demanding that Ben hand over Chromastone. He forces Ben to transform into Diamondhead, then uses a sonic device to shatter Diamondhead seemingly releasing Chromastone from the Omnitrix leaving Diamondhead alive but in pieces. Tetrax gives the freed Chromastone a crystal which its absorbs, then flies off into space. Kevin restores Ben to normal, who then demands an explaination from Tetrax. Tetrax reveals that Chromastone is the only one who can restore Petropia & its people back to life. He explains that the crystal he stole & gave to Vilgax (who used it to power the weapon that destroyed Petropia) was in fact a crystalized back-up for both his planet & its people. Upon discovering this Tetrax stole the crystal back from Vilgax & gave it to the freed Chromastone so it could use it to restore the planet. This was Tetrax's way of atonement for his part in Petropia's destruction. However, his theft of the crystal didn't go unnoticed by Vilgax who followed Tetrax to earth, demanding he hand over the crystal. Tetrax, Ben, Kevin, & Gwen are then chased by Vilgax who he believes are in possession of the crystal. Vilgax eventually manages to catch Tetrax demanding he hand over the crystal, but Kevin accidently reveals the crystals location. Vilgax flies off to Chromastone's location to discover the newly restored Petropia (Ben & Company race to stop him aboard Tetrax's ship). Angered that someone would use "his crystal" to restore a dead world Vilgax beats Chromastone who pleads for mercy, only to find Vilgax has none to give. Tetrax, Ben, Kevin, & Gwen arrive to see Vilgax beating the helpless Chromastone. An enraged Tetrax attacks Vilgax, but Vilgax shield protects him from the crystal projectiles. To Tetrax horror Vilgax removes the crystal from Chromastone (before he can revive the planet's people). However Vilgax discovers the crystal power is gone (thus it is no longer useful to him) and flies off in disappointment leaving behind the dying Chromastone. All hope seems lost, but the dying Chromastone generates a new backup crystal & hands it to Ben before fading away. Unsure of how to use the crystal Ben transforms into Diamondhead (who instinctively knows how to use it). Diamondhead uses the crystal to restore the Petrosapiens to life. Tetrax & a revived Chromastone (who is revealed to be Sugilite, Guardian of Petropia) thank Ben for his help. Tetrax decides to gives Ben & Co. a ride home. Before leaving Tetrax says he plans on settling the score with Vilgax (aware that his planet won't be safe unless Vilgax is stopped).
Personality
Tetrax witnessing the desctruction of Petropia
Tetrax is a battle-harden bounty hunter with a strong desire to do what is right. This is do in part to his involvement in the destruction of his homeworld, Petropia. Tetrax claims that he used to be a being who cared about no one but himself & that is what lead him to become a mercenary. It is only after Vilgax destroyed his home that he realized the affect his action had upon his own people. That experience made Tetrax into the heroic Omnitrix's Guardian He became. He even helps Ben become a better hero, teaching him how to use the Omnitrix more wisely (Ben later repays him by helping him & Sugilite revive Petropia's people).
He also deeply despises Vilgax due to their history.
Abilities
Being a Petrosapien, Tetrax possesses abilities similar to Diamondhead's: his body is made of organic, nearly indestructible, diamond-like crystals who can refract light, causing him to be immune to most laser and energy weapon. He has a limited habilitie to regrow lost limbs, as well as alternate his own physical constitution, making him able to do things such as fire crystal fragment as projectiles, turn his hands into blades, create structures of this crystals, etc. He's shown to master this power far better than Ben when he was ten. In addition to his natural powers, Tetrax is an experienced bounty hunter with excellent fighting and strategy skills. He is also equipped with various devices and weapons, such as an hoverboard, his own personal ship and a sonic device.