Someone who does a great job yet receives little or no recognition for that. an unsung hero usually refrains from claiming too much in return for his/her efforts.
"Mikel Arteta was considered to be Arsenal FC's unsung hero this season."
An unsung hero is a person who commits an extraordinaryact but does not receive recognition or proper respect. However, as the late George Carlin put it, "As soon as someone is identified as an unsung hero, they no longer are."