Ninja Village

The Ninja Village (忍者の里, Ninja no Sato) was a secret institution located in an unknown city whose sole purpose was the training of highly skilled ninja assassins. It is the hometown of Speed-o'-Sound Sonic, Flashy Flash, Hellfire Flame and Gale Wind. According to Flashy Flash, the village was all male.

All ninjas who graduated from the village possess extreme levels of speed and are highly skilled in both martial arts and swordsmanship.

Overview
The Ninja Village was a very horrible and utterly despicable place. This is because children under the age of seven were sent to this ninja village, where they were trained for 10 years, doing nothing except sharpening their assassination skills under extremely ruthless conditions. They were then sold off to factories of the underworld. Escape from the village is basically impossible, (although Flashy Flash managed to) and ninjas may only leave when they complete the curriculum and obtain permission to graduate. A day in the ninja village was 72 hours, with only 6 for sleep. The food is tasteless, and any information from the outside world is completely cut off. Any kind of emotions are not allowed to be expressed, and those who lose their emotions gained excellent ratings. Additionally, companionship and a sense of brotherhood among the classes is not allowed, and even talking among one another is prohibited. Instructors went so far as to say that if they ever crossed paths in the future after graduating, it was kill or be killed. Flashy Flash described it as a hellish environment. Despite this, many of the ninjas resent Flashy Flash for destroying the village, showing that they still respect the village.

The training regiment was extremely arduous. The ninjas were separated into five classes of training based on their skills. Class One was reserved for the most powerful ninjas, and people in this class received the most lenient training, while ninjas in Class Five, the class for the weakest ones, receive the harshest training to boost up their strength.

Hellfire Flame and Gale Wind are the graduates of the 37th class, Golden, and were among the top of that class, while Sonic and Flashy Flash are the sole survivors of the 44th class, with Flash being the No. 1 of the class. Flashy Flash is responsible for the deaths of the 44th Graduation Class and the rest of the entire village, known as The End, and all the ninjas of previous graduating classes regard him as a criminal. Although unconfirmed, it can be assumed that since Sonic was one of the weakest ninjas of his class, many if not all of the other ninjas that graduate from the village have S-Class capabilities.

History
Sometime in the past, the ninja village was founded by the man regarded as the most powerful ninja to ever walk the earth. From this point on, several ninjas were trained in separate graduating classes and were sent to factories in the underworld to be used as assassins and killing machines.

About 20 years ago, Flashy Flash was sold by his parents at the age of 5 to the Ninja Village. Throughout his childhood, Flashy Flash was consistently pushed to his limits and kept alive through specially formulated medicines. Eventually, he decided to purposefully perform badly in order to receive the harshest training in Class 5. Sometime during his training, he ran across Speed-o'-Sound Sonic, and the two bonded due to both of them being in Class 5. During a mission to dispatch a group of genetically-enhanced beasts as part of their training, Sonic expressed his desire to topple the corrupt administration of the Ninja Village in order to replace it with a more benevolent one and asked Flash to help him in which Flash gladly accepted.

Sometime during this period, the leader of the ninja village challenged Blast and was defeated. Although Blast didn't kill him, the leader was put in a coma, and the ninja village placed him in a regeneration chamber to speed up his healing.

Several years passed, and by the time Flashy Flash was 16, he'd long chosen to stop hiding his abilities and was ranked number one. Unbeknownst to the village, however, was that Flashy Flash had secretly begun to execute his plan of overthrowing the village administration. With his fighting abilities becoming formidable enough, Flashy Flash went on a rampage to fulfill the dream he and Sonic agreed upon, killing the instructors, staff, fellow members of his class, and basically everyone who was a part of the evil village. For some reason, Flashy Flash either could not or chose not to kill the leader of the village, possibly because his location was hidden from Flashy Flash or that the hero thought he would be too powerful to kill, even while in a coma. The only one who survived was Sonic, who had been poisoned by Flashy Flash to save his life from his rampage and thanks to the brutal training he endured being in Class 5 all of his life, he managed to survive.

After the village was destroyed, many ninjas of the previous graduating classes decided to band together and form a group known as the Heavenly Ninja Party. Their main goal is to kill Flashy Flash, who they all hate due to him destroying the village, and to welcome "him" (the leader of the ninja village) as their leader to kill the aging Blast. Once this is done, they plan to go out of hiding and take over the world with their superior ninjutsu skills.

Ninjas Arc
After Sonic is once again defeated by Saitama and is recovering, he is approached by the Heavenly Ninja Party. This party consists of ninjas from previous graduating classes of the village, including Gale Wind and Hellfire Flame, and reveal to Sonic their plans to kill Flashy Flash and Blast, return their leader of the ninja village back to his rightful place, and come out of the shadows to take over the world with their superior ninjutsu skills. They tell Sonic to lure Flashy Flash out, where they'll ambush and kill him. They also warn him that those who betray them will be executed. Sonic, being outnumbered, has no choice and complies to their plan.

Sonic lures out Flashy Flash, and the two begin to battle. As they fight, Flashy Flash is impressed by how much stronger Sonic has become. Sonic states that he's not the same fighter from several years ago as his strength has been improved a lot from his constant self-training to fight Saitama. Flash realizes this, and concludes that he has to use his ultimate moves to defeat him. It is at this point that the other ninjas attack Flashy Flash simultaneously with their weapons. The hero is able to dodge all the weapons, but is surprised by the number of people there and asks what this is all about. The fighters reveal themselves as ninjas from the village and their plan to kill Flashy Flash. But then, Sonic attacks one of the ninjas. He goes on to say that he could handle Flashy Flash, and for them to get out of his way. The ninja reveals his Ten Shadow Burial, impressing all of the ninjas watching him. The hero then decides to silence all the other ninjas, and their battle begins.

Trivia

 * Every known ninja from the village has a redundant name.
 * In Japanese, every known ninja's name consists of a word in kanji, followed by an English word with roughly the same meaning, written in katakana.
 * The name of the Heavenly Ninja Party (忍天党 Nintentō) is a parody of the video game company Nintendo.
 * There are other ninja villages in the world, such as the one Shadow Ring trained at. However, according to Hellfire Flame and Gale Wind, none of the other ninja villages are a match for theirs.