He's a lonely Akatsuki member he has no partner. He might have had a partner in the past since there were originally 10 members, but we all know Orochimaru messed that up.
Yeah, he has no partner, but he's kind of like two people in one. Almost like two-face from Batman.
Maybe Zetsu can split into two people?
He basically reminds me of those two twins from the Sasuke chase arc. Sakon and...his douche of a brother?
Tobi wasn`t Zetsu`s partner, he was Zetsu`s underling
zetsu has 2 voices. one that speaks in hiragana/kanji and the other that speaks in katakana. so he is teamed up with himself. i always thought it was his head talking to the fly trap. because even when oro was in akasuki their was only 9 if you only counted zetsu as 1 person. because oro was sasori's partner and he was replaced by dedara.