relator

noun

re·​la·​tor rē-ˈlā-tər 

: a party other than the plaintiff upon whose information, knowledge, or relation of facts an action is brought when the right to bring the action is vested in another: as

a

: the private person who brings a qui tam action

b

: a party who has standing and on whose behalf a writ (as of mandamus) is petitioned for by the state as plaintiff

relator then filed…a petition in prohibition requesting this court to prohibit respondents from transferring the funds

—State ex rel. Tate v. Turner, 789 S.W.2d 240 (1990)

see also ex relatione