| (マイト・ダイ, Maito Dai)
Duy spent his entire career as a genin, causing many to look down upon him. After twenty years of training, Duy perfected how to use all Eight Gates. He later had a son, Guy, who showed little progress himself as a shinobi, failing his first attempt to enter the Academy. When Guy began feeling sorry for himself, Duy scolded him for not having faith in himself, and ultimately taught Guy how to use the Eight Gates. One day, he and Guy met Sakumo and his son Kakashi in front of the Academy and assumed their sons would be classmates, only to learn that Guy flunked the entrance exam.
Some time after that, Duy rescued his son and his team-mates, Ebisu and Genma Shiranui, from the Seven Ninja Swordsmen of the Mist who had confronted the trio. Though his son argued that he was no match for the Kirigakure-nin, Duy reminded his son that he still had the Eight Gates technique and would use it under the very same condition of use he created: the protection of someone precious to him.
Duy was a proud and upbeat man. Like his son, he was overzealous, regularly using a "nice guy" pose, thanking people who were otherwise chiding him for his behaviour, or low rank. He believed strongly that one must have confidence in themselves in order to achieve anything. He also believed that one's true youth only ends when one turns their back on their beliefs and joy. Dai had a very strong relationship to his son, constantly pushing Guy to be his best and lifting his spirits when upset; a relationship that is parallel to Guy and his disciple, Rock Lee's. A committed man, he had no fear in facing his enemies.
Duy was a tall and well-built man with high cheek-bones, a somewhat bulbous nose, dark hair and thick eyebrows. His hair was cut in a bowl style that was combed to his right and also sported a bristling moustache with a small goatee and stubble which made up the rest of his beard. He wore a green jumpsuit, orange striped leg warmers, and a scarf around his neck.
Duy possessed a superb physical condition and capabilities in taijutsu. Having spent twenty years perfecting it, Duy mastered all eight of the inner gates. While admitting to it being his only achievement, Duy was able to fend off the entire Seven Ninja Swordsmen of the Mist with it and was competent enough with his abilities to teach it to his son.
Duy's sacrifice left a huge impression on Guy. He continued his father's belief of maintaining one's youth. Guy also formed a similar relationship to his student Rock Lee, as he did to his father, yet in reversed roles.
- Duy shared the same nickname as Kosuke Maruboshi. They are each known as the "Eternal Genin" (万年下忍, Mannen Genin).
- Duy's full name Might Duy is a pun on the phrase "Might Die", which is tragically ironic and foreshadowing considering he actually ends up dying after opening all Eight Gates.
- Duy was most likely made to honour Kishimoto's father, who passed away around the time the only chapter featuring Duy was released.
- (To Might Guy) "You know what a true victory is? It's not beating someone strong. It's being able to protect something important to you!"