At first, it refers to saber, one of the heroines in The Night of Destiny, because the dull hair on her head is conspicuous, and later it became her nickname. However, because Nero, one of the heroines in fate extra series, also has dull hair, saber in fsn is called blue dull (because the combat suit is blue tone) and Nero in fe is called red dull (because the combat suit is red tone).