Yui

Yui, the cat who is too strong for his own good. Last name Ashura. Most people mistake his name being Kenryoku because of some old myth.

He is the last of his species, called nekomons. Nekomons are an extraterrestrial race, equipped with overwhelming strength, senses, intelligence, and fighting skills. No being has ever defeated a Nekomon and lived to tell the tale. They have a strong connection to human children and tend to foster their creativity early on. Children enjoy seeing him because of his friendly feline face.

Every enemy to challenge him has been defeated in one blow. He's like the cat version of Saitama. He currently roams the Earth as a casual citizen and has chosen to be a hero to mankind. He's grown a feared reputation across the world. 

Yui has stellar combat skills, along with the ability to manipulate darkness and summon it at will. He can use this darkness as a sort of arm to grab things or attack people with. They can turn into the shape of anything he wishes; items, people, animals, or vehicles. However, it only works when shadows are present and is naturally stronger at night.

He has some extent of morals, although slightly different from humans' morals. For example, human morals (judicial system) would sentence several years of prison to someone who abused a child as punishment, then release them after they've served their time or had good behavior. But Yui's morals ensure that anyone who harms a defenseless child should instantly meet death.

His face remains still, smiling most of the time. It's said that when he's excited, his mouth opens slightly, but no one has ever seen the rare occurrence. It can also mean he's angry, so it's sort of terrifying for some people.

Through rigorous training, Yui has harnessed the power of every element and every state of matter. There is no ability, technique, or strategy he hasn't already mastered in his twelve centuries of living. (Meaning that yes, indeed, he has mastered every ability in every shonen anime to ever exist. OP, amirite?)

He willingly switches between villain and hero every few centuries, just to find new enemies and new fights. Being the strongest can get boring pretty quickly. Even when he's a villain, he never stops being gentle toward children.

Yes, he can defeat Goku. And Saitama, Yujirou Hanma, and any other strong character you can think of. The only one he can never rival is God and His angels.

Yui has a voice but doesn't use it much. It's not manly or heroic, it's like the small whisper of a little girl. This unexpected discovery has led people to believe he has no gender, or that he's a female. But option one would be correct. Akura, like many Nekomons, has a voice atypical of his pronouns. It's natural for their race.

Despite being named Nekomons, they're not actually cats, nor are they related to the felidae genus. Their faces just look like cats'.

Yui can transcend time and space at will, but he doesn't use this against his enemies. Just like many anime characters, it's his life goal to be defeated, so he wants to truly experience it when it happens.

No one knows how he got so strong, or how he is able to live for so long. He likes to keep his secrets.

He can turn big and buff when he gets serious, but has no overall forms.

He has a liking for koi fish and jasmines. Even with such destructive power, Yui always finds time to appreciate creation in all its beauty.

It's actually the same with all Nekomons. The source of their power is not training or natural prowess, it comes from their own mind. Every power they have is what they imagine. Everything they do is what they know they can, and everything they don't do is what they aren't confident in. Nekomons have the astounding ability to turn their imagination into reality. 

So even though Yui is a formidable fighter, most of his abilities were simply inspired by others. He has nothing but confidence and experience, and that makes him powerful. With this, he has no limits, and he's the only Nekomon who knows the true potential of his power. He never ages, never dies. He doesn't perceive the concept of aging, nor decay, therefore his being does not submit to it. And if he so chooses, he may even choose to disobey the laws of physics themselves. He's already done it with gravity and the laws of time before.

Only about three humans ever knew this in history, so they made it their life goal to achieve this power but have expired before doing so. This may be because Nekomons are a sacred, divine species, similar to angels. They're born with this ability. But just like humans, their bodies have fallen and they were given free will. Not many have been seen on the Earth, but they exist out in the universe. Some in the physical realm, some in the spiritual. They're quite fascinating creatures. 

Their ability to manifest thoughts into reality also relates to their strong kinship to children. Without their imagination, they're powerless. They can only see the world as an empty grey landscape with no emotion, no feeling. But when they use their imagination, everything becomes so vivid and they feel as though they're releasing themselves. If anyone could learn the power of the Nekomon, it would be a child with a powerful imagination.

So the world is lucky that Yui is so reserved with his abilities, because with such power, he could eradicate the Milky Way in a millisecond just by using his mind. But in the next second, he could perfectly recreate it as if nothing ever happened. Who knows, he's probably already tried it once.