Zetsu first appeared after the battle between Naruto and Sasuke in the Valley of End. He reported directly towards the Akatsuki Leader, informing him on the status of Sasuke Uchiha.
Zetsu later appeared with the rest of Akatsuki in the Land of Wind. While sealing Gaara's Ichibi, Zetsu informed Akatsuki that the Leaf ninja were closing in and so Pein created clones of Itachi and Kisame to stop them. He was then ordered to "dispose" of the bodies of the Itachi and Kisame impostors.
He's seen after the battle between Chiyo, Sakura, and Sasori, trying to find Sasori's ring. There, he found his subordinate Tobi with the ring and asking if he could join. After some arguing with himself, he allowed Tobi to join.
He's later seen after the battle of Deidara and Team 7. Here, he and Tobi recover Deidara's ring, but they discovered that Deidara was still alive.
Zetsu is then seen after Pein's battle with Jiraiya, one half wonders why the fight took so long while the other side said that Pein was fighting Jiraiya. Pein explains that Jiraiya showed up out of nowhere shortly after Madara ordered him to capture Naruto. One side of Zetsu asked if he could go with Pein to watch him catch Naruto, but the other side wanted to go watch Sasuke's and Itachi's fight instead.
After Itachi's and Sasuke's battle, he went to tell Madara that Itachi lost and they quickly went to retrieve Itachi's body and make sure Sasuke was ok. He then went to find Kisame, whom was having some fun with Suigetsu. Suigetsu and the rest of Team Hebi followed to rejoin with Sasuke. After Madara convinced Team Hebi to align with Akatsuki, Zetsu congratulated Madara on finally getting Sasuke but expressed grief that so many Akatsuki members died beforehand; Madara shared Zetsu's feelings.
After informing Madara that Danzo left for the Kage summit in the Land of Iron, Madara told the the white half of Zetsu to accompany Team Hebi, now calling themselves Team Taka, there to confront Danzo while the black half remained with Madara. Unknown to Team Taka, Madara also went to the Kage summit and told Zetsu to inform the Kages that Sasuke was there, and was apparently killed by the Raikage when he wouldn't say where Sasuke was. This caused a bit of chaos and Zetsu ended up helping Sasuke before Madara appeared to save Sasuke and Karin.
After killing Danzo, Madara left with Danzo's body and told Sasuke to return to base to recover. However Sasuke was interrupted by the arrival of Sakura, Kakashi and ultimately Naruto. White Zetsu ended up saving Sasuke, whom was hiding on Sasuke to watch his back as Madara ordered, and informed Madara that things were looking bad, so Madara teleported in to retrieve Sasuke and told Zetsu to meet up with his black half and check up on Kisame. White Zetsu finds Black Zetsu with Kisame's corpse and asks if he recorded everything, Black Zetsu says yes and that the Cloud ninja took Samehada with them, which means the plan worked; at Zetsu's whim, Kisame's corpse turns into one of White Zetsu's Spore clones and it's revealed that Kisame is hiding within Samehada to infiltrate the Cloud village.
After declaring a 4th ninja world war, Madara used some of the Bijuus' chakra to grow an army of Zetsus. After Kabuto formed an alliance with Madara, Kabuto kidnapped Yamato and used him to make the 100,000 Zetsu clones stronger. During this, White Zetsu remained by Madara's side relaying information to him, while Black Zetsu was sent to capture the feudal lords to use as bargaining chips against the ninja alliance. This has brought Black Zetsu into direct conflict with Mei and her squad.
The alliance captured one of the Zetsu clones and discovered that their cells are like Hashirama's. During the fight, one of Naruto's Shadow Clones came to help. During the fight, Black Zetsu sprouted many roots from the left side of his body which he used to trip Naruto and make him drop his Rasenshuriken, during this Chojiro was able to slice Black Zetsu in half while he was distracted, immobilizing him.
Zetsu as a whole is still a mystery, however he frequently uses "Mayfly," which allows him to fuse with the ground and trees, allowing him to travel great distances very quickly.
He has also displayed the ability to split himself in two.
He has used the "Spore" technique which grows into copies of itself and can even grow around the opponent and steal their chakra.
Zetsu can also grow roots from various places on his body to use in battle, and can also use them to communicate with other Zetsus.
Zetsu can use "Shape Shifter" to turn into perfect duplicates of anyone he has touched as well, and can use this technique on his Spore clones.
Zetsu is able to sense the chakra of people far away.
The Zetsu clones augmented by Yamato are physically larger and more muscular that Zetsu, and are still able to use Mayfly, however they are also able to open their bodies to look like venus flytrap mouths to capture the opponent and drain their chakra and have shown the ability to merge together to make a giant plant monstrosity.
Zetsu is also able to move during the Sealing ritual, a skill which he solely holds which makes him the lookout during the ritual. He also functions as Akatsuki's Hunter-nin, disposing of dead bodies by devouring them.
He has used Byakugan-like telescopic and x-ray vision which has not yet been explained, with the addition of being able to record what he sees.
Zetsu and his counterparts are plant-like humaoids and have shown to be incredibly resilient to damage.