The main thing we know about Juror #5 is that he grew up in a slum—and he's not especially fond of people who think everyone who comes out of a slum is a thief or a murderer. He's quick to defend himself against these types of people when he tells the other jurors, "I used to play in a back yard that was filled with garbage. Maybe it still smells on me."
And don't try telling him that it's nothing personal, because for him, "This is something personal!" Being from a slum, #5 is also more sympathetic to the defendant in the murder trial than the others are. Or at least he doesn't hold it against the defendant that he comes from a slum.
Juror #5 really comes in handy is when he tells the jury room that an experienced knife fighter would never hold a knife the way the murderer in this trial did. Instead, he says, "Here's how. Underhand. Anyone who's ever used one wouldn't handle it any other way." When asked if he's ever seen a knife fight, he answers, "Well, I have. You know, on my back stoop, the lot across the street, back yard. Switchblades came with the neighborhood."
It's #5's understanding of knives that adds a crucial piece of evidence for the defendant's innocence.