the one that isn’t
a person who is in a famous and small group (usually of 2 or 3) but far less popular compared to the other member(s)
It’s short for 有名/人気じゃない方 “the one who isn’t famous/popular”. It is often used to describe a member of a comedy duo who is not as popular or famous as the other. Sometimes, it’s also used to describe a thing that is not very popular compared to the others that have the same attribute (e.g. name/title).
Although he wasn’t very popular compared to the other members at first, recently he’s been very popular.