Out of all antagonists in the Titototter series, Shadow appears more than the other antagonists, which means he is the overall main antagonist.
Unlike the other Titototter antagonists, Shadow does not seem to get defeated or punished for his actions. However, in Sonic Gets Kicked Out!, he was kicked out of Sonic's house just like how Sonic and Silver were kicked out. But, shortly after Sonic and Silver left, Shadow came back to the house. He later left after a short fight ensued.
Shadow is an evil, mean, homicidal, unfriendly, aggressive, angry, violent, cruel, sadistic and uncaring hedgehog who likes committing crimes. He is clearly aware that he is evil and he even described himself as the definition of "Pure Evil".
However, despite being a villain, Shadow could possibly be an anti-hero since it appears that he cares about his pet lion cub named Simba who he calls Maria, he apparently didn't approve of Sonic hurting Amy's feelings which caused them to break up since he said "After what he did to her, it was almost cruel. Not even I would do that", and he was even willing to save Jet from dying which means he has redeeming qualities.