Nixels, are very, very small. So small in fact that they can be easily crushed by Seismo's large feet. They have little ears, arms, and legs. Their bodies are mostly black, with the exception of their circular, angry white faces. It's possible that the black outer shell of a Nixel is like armor since it's shown that they can pop right out and run around without it. It is unknown how they control the hands in the armor, as they have no hands when outside of it.
However, there is a member that is an oddity to this rule. He is Muscle Nixel, and is about the size of Major Nixel and is very buff.
Nixels are cute and small but vicious, annoying, crazy and destructive creatures who are spread all over the land of the Mixels. They live to tear down and break apart the world around them, including any cool combinations that the Mixels create. In fact, there is nothing the Nixels hate more than when two or three Mixels combine together and get super powers. So the Nixels have made it their mission in life to put and end to such crazy creativity all together. And the only way to do this is to hide and destroy the Cubits which enable the Mixels to combine in the first place. One Nixel can do little harm. But when they swarm together they can break apart a Mixel in seconds. So now you’re warned!
They consider themselves far superior to the Mixels, but are only able to say "Nixels", "Nix", "Mixels", or "Mix", blow raspberries, whistle, laugh, sputter, yell, inhale, exhale, and scream.