
Another character who is not given much credit is Cypher, from the original Matrix movie. Yes, he was an informant to the Agents, but it was his actions that led to the capture of Morpheus, and ultimately forced Neo to make a choice in saving him from the Agents. It was at that point that Neo finally realized he was 'The One'. Here is the conversation between Trinity and Cypher at the very beginning of the movie, where Trinity's phone line is tapped, because Cypher sells her out.
Cypher: Yeah.
Trinity: Is everything in place?
Cypher: You weren't supposed to relieve me.
Trinity: I know, but I felt like taking your shift.
Cypher: You like watching him, don't you? You like watching him.
Trinity: Don't be ridiculous.
Cypher: We're going to kill him, do you understand that?
Trinity: Morpheus believes he is the One.
Cypher: Do you?
Trinity: It doesn't matter what I believe.
Cypher: You don't, do you?
Trinity: Did you hear that?
Cypher: Hear what?
Trinity: Are you sure this line is clean?
Cypher: Yeah, 'course I'm sure.
Trinity: I better go
We also get the feeling that Cypher had a physical attraction to Trinity, and a lot of what he did was driven by jealousy as well, as he knew Trinity had fallen in love with Neo.