Osvaldo Cruz is a witness against both James King and Steve Harmon in the novel Monster by Walter Dean Myers.  Osvaldo claims to have been a part of the robbery and gives his testimony to the "facts" that he knows.  Osvaldo states that his involvement with the trial was solely based upon his fear of the others involved.

Osvaldo: He said he had a place all lined up. He said all I had to do was to slow anybody down who came out after them.  I was going to push a garbage can in front of them. He (Bobo) said that he and King were going to go into the store and do the thing. Steve was going to be the lookout.

Osvaldo is giving his testimony on the trial as part of a deal from the government.  His testimony of his involvement, and of Steve's, comes into question because of the deal he has made.  In the end, it comes out that Osvaldo has been involved in many different crimes, one being a gang member and initiation rites where he had to cut someone in the face to "leave your mark on somebody".

Basically, Osvaldo turns out as a very bad witness for the defense.