pursuer

n. follower, chaser; plaintiff, claimant (Law)

A pursuer in Scotland is the party who initiates a lawsuit before a Court of Scotland. The term is the same in civil and criminal proceedings. The pursuer is seeking a legal remedy, and if successful, the court will issue judgment in favour of the pursuer and make the appropriate court order (eg. an order for damages or find the defendant guilty and punish them.)
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Noun
1. a person who is pursuing and trying to overtake or capture; "always before he had been able to outwit his pursuers"
(synonym) chaser
(hypernym) follower
(hyponym) bounty hunter
(derivation) pursue, follow
2. a person who pursues some plan or goal; "a pursuer of truth"
(hypernym) person, individual, someone, somebody, mortal, human, soul
(derivation) prosecute, engage, pursue

(n.)
One who pursues or chases; one who follows in haste, with a view to overtake.
  

(n.)
A plaintiff; a prosecutor.
  

     usurper