There's nothing much known about Coconut Sid, though Sweet Dick Willie's comment that he "got off the boat too" seemingly indicates that Coconut Sid was from a different country.
When Radio Raheem is getting choked to death by the police, Sid is the first to notice and shouts at them: "Stop it! You're killing him! You're killing him!"
When ML goes to the Korean grocery store, Coconut Sid tells him to leave them alone, saying that they're alright.