Mentors are assigned to train their District's Tributes before the Games begin. They have been chosen based on skill and their loyalty and willingness to help.
They are the winners of the previous Hunger Games.
The list is as follows:

District 1: James Moriarty

District 2: Daryl Dixon

District 3: Remus Lupin

District 4: Crowley

District 5: Spock

District 6: Captain Jean-Luc Picard

District 7: Dr. Emmett Brown

District 8: Bellatrix Lestrange 

District 9: Ace Merrill

District 10: Bobby Singer

District 11: River Song

District 12: Rufus

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