Who am I?
Your sixth Finger
I run faster, You run slower
My biggest enemy is my doppelganger
Choo Choo(Resemblance)
I don't weigh much
Touch me and I will hurt you back
More hints will be added if it is hard, I don't think it's hard though.
P.S The last clue was because i think it give you a vast opportunities without the last clue , so trying to make the circle smaller.