secondant

second (attendant of a duel or boxing match standing in for a contestant)

1798, a letter written in London on the 21 of May, published in Матеріалы, book 3 (1890), page 286

Upon which Mr Tierney did not say a word; however, the next day he sent a cartel [a written challenge to a duel] to Mr Pitt, who accepted it and went the day before yesterday to the specified place, as did Mr Tierney for his part, each accompanied by his second.

assistant, coach, trainer