|First Appearance||Greninja: Training with the Master|
|Race||Pokémon (dark/water type)|
Greninja is, in an overall sense, a typical strict mentor, usually mysterious, firm and even a little bit cold. Sometimes his judgement can be questionable, making random sneak attacks to test his 'students' on their skills. He tries to keep his main information secret, and those times he does share, he leaves vague and confusing for the others to unravel themselves. Greninja is a straight-minded mentor, sticking to tasks and missions without distractions.
One day, Greninja was summoned by Arceus, and was given the mission to keep track of Korosensei, and he was sent off. While traveling through the forests near Tokyo, he came across Nagisa, and performs a sneak attack to test Nagisa's judgement, which he claimed he needed work on. After a brief introduction, and direction, Greninja headed off to the old campus.
Upon arriving, he used his water Shirikun to slice off Korosensei's whistle. After looking over the school he arrived to, he threatened Korosensei with his blade, testing his reflexes. He did similar tactics to Luffy and Naruto, giving his thoughts towards each of them until Nagisa returned, which he didn't hesitate on sharing to the others that he told them. After being told about the old campus, he warns Korosensei to not harm anyone, and then leaves.
Sometime later, he finds Nagisa with Twilight in the forest, and tried another sneak attack on them again, which he claimed he'll keep doing if they're not prepared. After a brief introduction to Twilight, Greninja gives a vague warning of Giratina's monsters, and leaves.
He remained in the 3-E classroom until Naruto requested for some training, which he agrees to after some thought. During a practice fight outside, he gives some basic fighting tips, which include to drain your opponent of energy before finishing them, which he used on Naruto. Afterwards, he watched Korosensei congratulate Naruto, which he found as confusing and even foreign, as he went back inside.
When Jerome found Class 3-E, Greninja was the one who let Nagisa go off on his hunting trip, despite Korosensei's complaints. However, he also said he can't go with him, trying to keep his presence a secret. After Nagisa returned, Greninja had sensed the pressence of Misao's spirit, and called upon Nagisa, Luffy and Naruto to go and investigate the incident, as apart of a test to see if they're able to handle such situations in the future, becoming impressed to Nagisa mostly.
After the event, Greninja and Korosensei had spoke for a while, and Greninja revealed that Kaibutsu and Misao weren't the threat he was sent out to guard against. As he did, he found that Ren Hōsen had arrived to the school, and quickly began to have some thoughts about her, mostly due to how strong she was as he kept an eye on her throughout her time there.
Greninja eventually left Class 3-E for a while to handle some other matters, until he returned to find Karma Akabane. He spoke with him, but finds an Angel Fleet badge in his possession, immediately demanding where he got it from. However, he didn't get his answer, and soon left again. After a few days of absence, Greninja finally decided to tell them the entire story, but told Nagisa, Karma, and Ren to meet him within the forest. When they met him there, Greninja explained to them about Giratina's threat, and that they possessed the Multitype. Greninja confirmed that they can relax until he can get more information, and left them on that note.
Throughout the following summer, Greninja mainly done his duties until Korosensei and the students had returned. Greninja was angered, not knowing they were going to just leave, until Karma revealed he unlocked his Multitype, Kasai. Greninja calmed down after hearing that. Nagisa asked him later about the Multitypes, Greninja agreeing after hearing that Nagisa had met Arceus. Later, he explains to Nagisa about how to unlock the Multitypes, though also saying he'll be unable to help until that happens. When Weed told him about leaving, Greninja was against the idea, thinking they all should stay together, yet was convinced to let him go.
Greninja is an expert in ninjutsu, knowledge of the art surpassing most users. His various techniques, and weapons were based off master ninjas, nearly all self-taught during his years in the Angel Fleet.
Naturally fast, Greninja's speed and agility allows him to move around at lightning speed, and maneuver with ease to dodge his enemies.