Shadow the Hedgehog

Shadow is a fictional character from the Sonic the Hedgehog series. He is an anthropomorphic black hedgehog and the arch-rival of Sonic the Hedgehog. Shadow was created as the "Ultimate Life form  by Gerald Robotnik, using the DNA of Black Doom, and is the final result of Project Shadow. His purpose was to provide ways of developing cures for deadly illnesses that had no cures, namely for Gerald's granddaughter Maria Robotnik, but he was captured and put into stasis by the military when he was deemed a threat to humanity.

Fifty years later, Shadow was released by Dr. Eggman to help him conquer the world. Initially, Shadow sought to destroy the earth to avenge Maria, who had been killed by the military, but was persuaded to help save it from Gerald's doomsday plans. In the process, Shadow lost his memories and he sought to uncover his past, which brought him into contact with the Black Arms. Ultimately, Shadow thwarted Black Doom and saved the world. From there, Shadow continued his mission to protect humanity by becoming a special agent of G.U.N..

Shadow is sharp-witted, always on edge and has a strong sense of purpose, willing to accomplish his goals by any means necessary. Essentially a dark version of Sonic, Shadow can match him in both speed and skill, and possesses the ability to use Chaos Control. While preferring to work alone, he will help Sonic and friends when necessary.

According to an official poll, Shadow is the second most popular character in the series, behind Sonic himself.

Shadow and the Snow Queen
In Shadow and the Snow Queen, Shadow serves as a very important character, hence the name "Shadow and the Snow Queen". He often adventures with Elsa of Arendelle, the two going tp often dangerous places or situations. However, despite them disagreeing over quite a few things, it is possible they may have feelings for each other. Currently, they are only allies and (perhaps) friends.