I see that this thread is old, but it is apparent that the image in question has some form of anti-aliasing around it. I am responding in hopes that any others with similar questions will find this answer. Perhaps a mod would like to change the header to reflect the user's particular issue?
If you pasted a transparent PNG over another layer such as your pink color, it will be a matter of the transparent layer's edges having varying opacity and blending in a gradual fashion at pixels which fall on the edges of the image. In this case, you need to remove the anti-aliasing that is already in the image. There are many ways to go about this, but no specific tool of which I am aware.
If you only have a single layer in which you've filled the negative space (empty area around the character) with a fill bucket tool, you likely have anti-aliasing enabled on the fill tool. This will blend the edges of the character with your chosen color, and the anti-aliasing or edge blending function must be disabled. Refer to the support for your chosen software.