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Looks can be deceiving,

And they certainly were with me.

Betrayal and lying,

People dying,

Makes no difference to me.

I'd do anything to help him get that wretched boy. 

That would make my master's empty heart fill up with bubbling joy.

Much in common I have with him,

We killed the people we "loved" or not.

I disguised myself for this man,

I'm doing everything I can.

For years and years I endured it all

Just to see that Potter's life come to a sudden halt.

Who Am I?

The answer is Barty Crouch Jr.

at first I was thinking that it was peter pettigrew since he disguised himself so greatly but no.