Could be - perpetrator, victim, witness or accomplice.

Depending on the circumstances, they could be an accomplice, co-defendant, or witness. If they participated in some way, they are likely an accomplice or co-defendant.

An accomplice is a co-accused with other persons in commission of a crime. While one of the accused persons turned to a prosecution witness on the condition of giving him pardon, he is an approver.

It depends upon your culpability. Are you simply a witness, or are you an accomplice that "aids and abets" the crime?

Evidence/Proof beyond a reasonable doubt that you did it. Usually proven by bhe burden of evidence, witness or accomplice testimonry, or by handwriting examples.