"Unlikely friendship" refers to a relationship between two people who are very different from each other and would not usually be expected to become friends. Full definition
But it makes up ground with an ensemble of swell performances, an ear for everyday banter, and a nice eye for the way unlikely friendships develop. (deep-focus.com)
As unlikely a friendship as it was but not all together surprising. (bookbrowse.com)
From this dubious beginning they form a most unlikely friendship which we see develop from their alternating points of view over 3 months. (bookbrowse.com)