odd man

noun3

1. •
   In a group having an odd number of people, someone with the casting vote; an arbiter.
2. •
   Someone who does odd jobs.

   • ‘My father always kept a dog-cart, and we had three servants. We had a cook and a housemaid and an odd man.’ — 1915
3. •
   A man who trains in company with a boat's crew, so that he can take the place of anybody who falls ill.