Little to nothing is known about Nekketsu. All that is known is that he was once approached by Nightmare Enterprises and he accepted the offer. He was sold to King Dedede after a teacher he had ordered, Mr. Chip, was doing nothing to harm his students. King Dedede took him to school the next day, revealed Mr. Chip's backstory in an attempt to demonise him and then sicked Nekketsu on the class by throwing a blue orb, which exploded on the floor. After a dramatic reveal, Nekketsu turned around and laughed, his eyes flashing red.
Battle with Kirby
After destroying a desk with a bokken, Nekketsu started to attack Kirby repeatedly. Mr. Chip shouted at Nekketsu to not touch Kirby before running to attack him. This attempt was futile as Nekketsu knocked him down. Tiff tried to tell him to run away while he still could, but Mr. Chip insisted on staying to protect his students. Nekketsu then hit Kirby out of a window before chasing after him. After he messed up his landing, Kirby was able to inhale his bokken and become Sword Kirby.
After losing his bokken, Nekketsu drew an energy sword similar to a lightsaber to fight Kirby with. The two charged at each other, resulting in Nekketsu launching Kirby onto the school's roof. The monster stretched his arm after Kirby, but couldn't quite hit him due to the latter's agility and the former's limited sight from the distance he was at. Shrinking his arm back to normal, he furiously ran after Kirby and joined him on the school rooftop. The two ran at each other once more and duelled for a few seconds before Kirby escaped by jumping off of the school's bell tower. Nekketsu quickly gave chase but this proved to be his undoing, since this distraction gave Kirby enough time to charge up a sword beam. He swung at Nekketsu, who managed to block the attack with his energy sword. Turning upside down, Kirby fired one more sword beam. This one was so powerful that it destroyed Nekketsu's blade before cleaving straight through him. After a brief delay, Nekketsu tumbled apart vertically, somehow still screaming, before crashing through the school rooftop and exploding, taking the building with him.
Nekketsu is a very humanoid-looking creature. He has gray hair and gray eyebrows, and he has sunken-in yellow eyes. His gray hair is held back by a bandana with red, black, and white colorings that run along it in a zigzag pattern. He wears an evil grin on his face (which looks similar to early Shonen Jump faces), and his mouth is large and full of pearly white teeth. He wears blue shoes and is very well-dressed, wearing a pink shirt with a green necktie and a purple tuxedo over his pink shirt, and he wears white gloves. A red yin-yang is painted on the back of his tuxedo. Nekketsu comes complete with a bokken that he can whack anything and anyone with, as well as a blue energy sword with a golden handle, which resembles the Beam Sword item in the Super Smash Bros. series.