Why do I have a thing for evil doubles?
This includes the character evil double even if not a technical one, such as Shadow the Hedgehog(yes, I will admit he is an evil double of sorts; it was part of the whole concept of SA2 to have an opponent with similar qualities yet on the opposite side), or technical evil doubles such as Scourge the Hedgehog and Evil Crash.
I just have a thing for them, I don't know why, and I always advocate for their presence in future titles after they appear.
I also like when characters get in touch with their evil side, or when bad guys turn good.