This dude is a chef who used to be Socrates' personal attendant and cook. Socrates trained him into peaceful warriorhood, similar to how Dan is being trained during the novel. Joseph specializes in beautifully preparing uncooked, natural food at his cafe, and helps Dan learn how to breathe properly (4.297-302). Joseph is an all around good guy, and very likable.
When his cafe is suddenly destroyed in a fire, he takes it with peaceful-warrior equanimity. He even says it doesn't matter where he goes with his life next—a very Socrates-esque way of looking at life (see Theme 4: Dreams, Hopes, and Plans). But he suddenly dies from leukemia (4.396). That's Joseph in a nutshell.
So why does he matter? He basically the example of a peaceful warrior (other than Socrates or Joy). He's friendly, and his restaurant burning up and his death serve as teachable moments for Dan. Plus, he's oddly likable, which is fun to read.