It is stated that he is of the same species as Sixsix, but is "11 times more dangerous" (as 66+11=77). Like his younger brother Sixsix, he does not speak English, even if he talks to aliens who do, but they are shown to be able to understand him. In an extended episode of the first series, Ben 10, it is mentioned that Sevenseven is Sixsix's older brother, although Sevenseven isn't genetically mutated like Sixsix is, as he has dark grey skin and black-and-white eyes, and not a light grey skin and robotic eyes, like Sixsix.
Sevenseven is like a non-mutant cyborg, but it can be seen that he was experimented on, as he had nano-robots infused into his hands so they could transform them into weapons, like some androids do. He can transform both of his hands into cannons that fire laser discs, energy beams, and laser machine gun bullets, and a buzz-saw, and has wire cannons that fire laser discs and machine gun lasers, discs, turrets that fire lasers and energy saws and homing heat-seeking missiles, grenades and short-range flash-bang grenades.
In the episode X = Ben + 2, Sevenseven kidnapped Attea, the daughter of the Incursean emperor Milleous, and holding her for ransom. When the ransom was paid, however, Attea offered him twice as much to help her overthrow her father. During a fight with Swampfire, Sevenseven's helmet was damaged, forcing him to retreat.
He reappeared, after a long time, in the Ultimate Alien episode, Hit 'Em Where They Live, teaming up with Zombozo the clown, Charmcaster the magician, and Vulkanus, the bounty hunter to take revenge on Ben, for their defeat, by almost killing his Mom, kidnapping his Dad and Gwen and fighting Max. He was seen targeting Sandra Tennyson but fought Max Tennyson instead when he showed up and flew away when Max sticks a knife into his jetpack, making it malfunction.
In The Big Story, he fought Gwen, Kevin and Rath. He ended up being tied to a street lamp, and then being left there.
Sevenseven appears in the video game Ben 10 Ultimate Alien: Cosmic Destruction on the Devil's Peak level, part 2. He is one of the bounty hunters hired by the Evil Way Big on Earth who are hired to take the Potis Altiare, an alien artifact created by Azmuth's ancestors that can upgrade the Ultimatrix. After an attempt to defeat Ben by shooting him, he fights Ben himself but is easily defeated, but he escapes. In the DS Version, Ben encounters him at the top of the wall in China.