Lurch

"It's nothing big. I just need a few refreshing swallows of that warm, red, delicious blood that runs through your body. I haven't killed you yet, so you could at least spare me a taste... right?"

Backstory: Lurch is an earlier version of Sonic similar to a test prototype. Its purpose was to run through the levels of Sonic the Hedgehog 2 (the game) and make sure the levels were passable. While running through the Chemical Plant Zone, he fell into the purplish water and was unable to swim back up, sinking to the bottom. The developers saw this problem and added moving platforms to the level later in life to make it easier for Sonic to get back to the surface if he happened to fall during gameplay. But before then , giant leeches (which showed up due to various bugs in the early version of the game) noticed Lurch, attached themselves to its head by rearranging his code , and swam him up to the surface just as he had run out of air. To return the favor for saving it , Lurch reluctantly agreed to help feed the leeches for the rest of its life. After Lurch's transformation was noticed by the playtesters, he was immmediately trashed, soon to be replaced by a newer prototype who'd go on to be the " Sonic " we all know and love . But the EXE managed to escape eventually , traveling to different universes and dimensions in and out of the Sonic world and killing victims to drain their blood.

Appearance: Lurch is similar to Sonic in several ways. It still wears Sonic's iconic shoes and gloves. However, it is paler and darker compared to Sonic. Not to mention the giant leeches attached to its head. Lurch's eyes are a white X overlapping a red circle. It also secretes a dark goopy liquid that's usually running down the sides of its ears, its cheeks, and the bodies of the leeches. 

Personality: Lurch is usually a quiet creature. It will either approach its victims in a friendly manner — asking to hang out with them and be their friend — or stalk them quietly until they're alone. After getting comfortable enough with his target, it makes its move.

Lurch will pursue said target, trying its best to leer them into water since Lurch is an excellent swimmer thanks to the leeches. It is not as good of a fighter on land, but retains its speed from the game testing. The speed is utilized more efficiently in the water however.

Additional Facts: 

Lurch will not kill anyone that doesn't have blood in their body (for obvious reasons).

It doesn't want to kill anyone, but if it doesn't feed the leeches regularly, they'll kill it .

Lurch will kill other EXEs but only if absolutely necessary. It'd prefer not to get into a serious fight.

It has a leech that lives in his mouth rather than on his head like the others. 

Its body feels extremely slippery and oily rather than furry like Sonic. 

Unlike regular leeches, the viruses attached to Lurch's head are always hungry and need to consume blood every few days. They can go up to a week before getting too angry.

With its transformation, Lurch gained some common leech-like behaviors. Some of which include but are not limited to: residing in the water for the most part, being extremely responsive to stimuli, breathing underwater, and detecting changes in vibrations/water temperature/light intensity, etc.