Zetsu (ゼツ, Zetsu) is a member of Akatsuki and one of Tobi's most loyal subordinates. The black half is known as Black Zetsu (黒ゼツ, Kuro-Zetsu), while the white half was known as White Zetsu (白ゼツ, Shiro-Zetsu).
Accordingly with his two differently coloured halves, Zetsu has a split personality. In the Japanese version of the manga, Black Zetsu only uses katakana for okurigana and furigana while White Zetsu uses kanji and kana regularly. This is often conveyed in the anime through the use of different voice actors. The two halves also converse with each other, and occasionally differ in opinion (Black Zetsu is more serious and knowledgeable while White Zetsu is carefree and easygoing). Despite these individualities, they are still able to communicate through their thoughts and function as a single entity.
Zetsu is seemingly very trustworthy, being entrusted with knowledge of both Pain and Tobi's true identities. He has apparently been working with the latter for some time, as Tobi first appeared as his "subordinate" and Zetsu later commented that Tobi's plans had been in the works for a long time. The character showed some lust for money when he quoted Kakuzu on his principle that money is most important. However, whether or not this applies for both personalities is as of yet a mystery to be dealt with. White Zetsu is also very confident in himself as he was willing to fight Naruto despite knowing that Pain had lost in battle against him.
Zetsu's main role within Akatsuki is to function as a spy, and often performs duties that are similar to a hunter-nin. He is able to control his body and take part in the tailed beasts sealing ritual simultaneously which, in addition to being unique to him, allows him to act as a lookout while the sealing takes place. When an agent to Akatsuki, or possibly even a member, dies, Zetsu is sent to devour their bodies so as to not have their secrets revealed, which adds further to his venus fly-trap theme. Zetsu is also the one sent to retrieve the rings of dead Akatsuki members. Due to the role he plays, Zetsu is the only member of Akatsuki to act without a partner on a regular basis. As such, he operated on his own, often moving between the different locations that the other Akatsuki members were located. This resulted in the members reporting directly to him rather that the figurehead leader Pain. Zetsu would in turn report this directly to Pain as well as the actual leader, Tobi.