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, although Sevenseven isn't mutated like Sixsix is.

Ultimate Alien
He reappeared in the Ultimate Alien episode, Hit 'Em Where They Live, teaming up with Zombozo, Charmcaster, and Vulkanus. He is seen targeting Sandra Tennyson but fights Max Tennyson instead and flies away when Max breaks his jetpack.

Trivia

 * 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 his younger brother Sixsix, probably because 66 + 11 = 77.