The phrase "unlikely ally" refers to someone who you wouldn't expect to become your friend or supporter, but ends up helping or working with you. Full definition
The two clash at first, then become unlikely allies, then friends, as they get closer to core of the criminal operation. (filmnoirblonde.com)
He finds a most unlikely ally when a Monster appears at his bedroom window. (du-hd.com)
The episode also hints at character interactions we might not have seen before, mixing up the teams and forcing them into unlikely allies. (goodereader.com)