The Nitro Blade User

The Nitro Blade User (often shortened to TNBU) is a fanmade character made solely for the purpose of killing Skylings, similar to how the ALL kills Luminous.

Their whole point is to be an unknown species and character, so that they cannot be targeted by other people relying solely on this information. They are an enigma.

Appearance
They are a humanoid species wearing a dark hoodie, sometimes portrayed as a cloak. Their hood covers their eye area, so that it is unsure as to what species they are (the tentacles (if any), eye mask, ears, etc. are all covered).

They wear dark gray sweat pants, and their shoes range from boots to sneakers.

It is confirmed that they are NOT secretly a human that survived the tides, meaning they are a species that evolved to be humanoid.

Personality
They are neutral to other species, as they target Skylings alone for what they've done. After being mocked for being unable to do anything when Skyling assassins killed their relatives, they've built up anger towards the species and sworn revenge on the whole species. They are cold and merciless to Skylings, even if a Skyling tries to prove that they are good and/or innocent.

Powers
While the humanoid themselves is not known to contain any power, they wield the Nitro Blade, a weapon that Corrin uses in Permafrost. How they got ahold of this blade is unknown, but it is also confirmed that they themselves are NOT Corrin.

They are later given flight after killing 99 Skylings and Queen Calliope. The wings are magical, so they can appear and disappear at will.

Role
They kill Cloud in Skyboundroad when Rider doesn't finish him off. Cloud asks "Where do you think you're going, hot shot?", to which TNBU replies with "You got two things wrong about me. One, I'm cold. And two, I use a Blade, not a Gun."

They then proceed to stab Cloud in the heart and kill him, cutting off his wings as well.

TNBU also tried to kill Ban, but only succeeded in cutting off one wing. This made her unable to fly, but she still lived.

TNBU later killed Queen Calliope for her crimes and took her Queen's Soul (even though she was not of royal blood), allowing them to gain Nitro Wings and fly.