Kosuke Maruboshi

is a genin-level shinobi of Konohagakure and is known as the and ,

Background
Kosuke has been a genin for over fifty years, thus the reason others called him the Ten-Thousand-Year Genin. When he was still a child, he made a reckless decision that cost him the lives of his team-mates. Feeling ashamed of this, he felt he was not worthy of being promoted beyond the rank of genin. Despite this, he continued to train arduously to improve his skills, something that was insisted upon him by Tobirama Senju. During his time as a ninja, he had served under the Second, Third, and Fourth Hokage and now apparently even the Fifth Hokage. At other points, he fought alongside the Legendary Sannin and Konoha's White Fang.

Personality
Kosuke has a very calm, collected and kind persona which seemingly comes from his years of knowledge and expertise, often seen smiling and hardly letting anything bother him even while on missions. This demeanour however, should not be taken as him being lazy or uncaring as he was quick to action when the team was being attacked and with his expert survival skills. He is also quite loyal, holding closely to the Will of Fire belief shared by the villagers with which comes his great sense of duty. This is seen in his willingness to sacrifice himself to allow Naruto to escape to warn the village and when he got angry at the Iwagakure jōnin kunoichi that had sacrificed her genin-level ninja during the battle. Because of what happened during his youth, Kosuke seemed to put little value on his life, putting the village's well-being ahead of his own. This mentality has seemingly changed after he met Naruto Uzumaki.

Appearance
Kosuke is an elderly man with his once black hair now greyed which he wears in a short ponytail. He wears a mesh armour shirt under a short brown sleeveless kimono as well as grey pants. He is usually seen with a smile on his face. He carries a metal pan that he uses for multiple reasons ranging from cooking in while out on missions to a weapon during battle. He also carries a ladle and a sword sheathed horizontally across his lower back.

Abilities
Even though his official ranks is that of genin, he is easily skilled enough to become a jōnin; a promotion he declined time and time again. He has shown considerable taijutsu skill, able to use only his cooking-ware to defend and maneuver against enemy attacks. He is also a skilled kenjutsu fighter and wields a sword and a hidden blade concealed in his prosthetic left leg, both of which he can use to strike with deadly precision. He also demonstrated the use of a unique technique which incorporated genjutsu along with kenjutsu to confuse and petrify his opponent, making it easier to strike them down. He also learned the Shuriken Shadow Clone Technique from the Third Hokage which he uses with similar proficiency.

In addition to combat skills, Kosuke is a highly capable survival expert and cook.

Nature Transformation
When he was younger, he was taught many techniques by the Second and Third Hokage. His training under the Second has given him highly proficient skill in high caliber Water Release techniques, allowing him to use techniques such as the Water Release: Water Dragon Bullet Technique and Water Release: Water Encampment Wall. He is also seemingly capable of using Earth Release techniques such as the Earth Release: Hiding Like a Mole Technique.

Konoha History Arc
He was placed on a team alongside Hayama Shirakumo, Tekuno Kanden and Naruto Uzumaki to guard the borders of the village after Orochimaru and Sunagakure's invasion of the village. While on the mission, they realised that their borders were invaded by several Iwagakure shinobi. After being left by both Hayama and Tekuno, to report to the village what had happened while they held them off, they were confronted by an elite Iwagakure team that had broken through the defensive lines. Clashing with them Kosuke is able to defeat them with his Leaf-Style Willow technique.

After being confronted by another battalion of Iwa-nin, Kosuke sends Naruto ahead to warn the village while he held them off. He successfully defeats all the subordinates of the group as he directly clashes with the leader who mistakes him for the leader of the team as well. After Naruto returns claiming that he could never abandon a comrade on the battlefield, Kosuke declares that he would not die nor never allow Konoha to be besieged by someone who would so willingly sacrifice their subordinates. With this he defeats the Iwagakure kunoichi who seemingly faints from expending so much chakra.

When the village was being rebuilt after the Invasion of Pain, he remarked that Minato had left behind a fine son, showing that he was one of the few people who was aware of Naruto's lineage.

Adventures at Sea Arc
During the rebuilding of Konohagakure, Kosuke came across Team Asuma whom he greets fondly, telling them that he owed a debt to them for saving him many years ago. After retrieving valuable information from the Three Brothers from Iwagakure, Kosuke rushes back to the village to relay the information. He is however overpowered by the three brothers and when seemingly trapped, Team Asuma arrives as back up and saves him with their Formation Ino–Shika–Chō. After thanking them he tells them about the first time the formation had saved him during the Invasion of Konoha, causing the three friends to realise that their fathers were amazing. As he leaves, he tells them not to worry as good shinobi aren't made in a day, and that he'd be more careful so they wouldn't have to save him again.

Trivia

 * Though actually called the this monikers have been subtitled as the "Eternal Genin".
 * Kosuke is the first anime-only character ever to appear in the series in different arcs.