SevenSeven

Sevenseven is a villain in the series, first appearing in X = Ben + 2. It is stated that he is of the same race as Sixsix, but far more dangerous. Much like Sixsix, he does not speak a word of English, even when talking to those who do, but other aliens have been shown to understand him. In a marathon of the first series, it is mentioned that Sevenseven is Sixsix's older brother.

Ultimate Alien
He will reappear in the Ultimate Alien episode, Hit 'Em Where They Live, teaming up with Zombozo, Charmcaster, Rojo, and Vulkanus.

Trivia

 * He's shown to be a coward, as he flinched from the battle after one shot to his eye by Swampfire, whereas Sixsix would've decked Ben. This could be because of Swampfire's smell.
 * He could have the same weapons as Sixsix though he didn't show much.
 * In the Pop-up edition of Ben 10, Sixsix gets his name from the weapons on his wrists, with six holes to launch small missiles, Sevenseven could have the same reason.
 * Gwen said that Sevenseven is eleven times stronger than Sixsix, reference to 66 + 11 = 77. Which makes him Sevenseven.