You're probably getting a lot of "it looks like [character]" comments because, let's face it, a human-looking character with a demonic-looking/behaving arm is common, especially if it's a defining character trait.
Ashitaka Scar from Fullmetal Alchemist That guy from Prototype (game) Vincent Valentine from Final Fantasy VII Nero from Devil May Cry Various arms from Bioshock: Infinite Nightmare from Soul Calibur Allen Walker Qilby from Wakfu Too many DA OCs... Main character from Ninja Gaiden 3 Inuyasha posessed by So'unga That guy from Naruto with all the eyes in his arm Tetsuo Elfman from Fairytale
I agree with the 'knee-jerk' comment, and I don't think that's always negative. Your characters might have a resemblance to other characters because let's face it: anime and manga build upon one another. But you twist the personalities and actions of the characters in SIN and it makes everything completely different! I hope the characters continue to be even more developed and start to write the chapters themselves. It's great so far