I read a sentence in a fiction: "He and Charlotte had met in their sophomore year of college, at an intramural volleyball match. She was a ringer, the setter for a team of frat boys." What is "ringer" and "setter"?
Each volleyball team has one player called the "setter", so she could be "the setter for a team" if she was a volleyball team member. Usually the "setter" is not tall (cannot jump above the net to block or spike) but is very very good at ball control. They are on the team because that skill is so valuable. In most plays, that person "sets" the ball -- they toss the ball to the perfect spot for a "spiker" to hit it very hard and downward ("spike it") at the opposing team. Since their job is to "set" the ball, they are called the "setter".
A "ringer" can be someone who doesn't belong there. Often it means a professional player, who is playing on an amateur team. That is against the rules. Perhaps Charlotte is playing on a boy's volleyball team, pretending to be a boy. That makes her a "ringer". Most "sport" level volleyball matches are all-male or all-female.
But it may be that this intramural (two teams from the same school) match allows male and female players. If so "ringer" has some other slang meaning here.
Dojibear's explanation of ringer is a likely one but note that it can also mean (WR dictionary) "something or someone looking exactly like another". We don't have the background to know if that's the intended meaning.
Ringer is used to indicate someone who has an unexpected/hidden level of skill. (It's not the same as but is related to this from dojibear: Often it means a professional player, who is playing on an amateur team.)
When the opposing team/player sees that person, they think they will have an easy match. The person doesn't look like they have high skill. In this case, that's probably because she's a girl playing with and against boys. The opposing team assumes she will be the weakest player. But she has very good ability as a setter (she probably played volleyball on a girls team when in high school) so it is only after the match starts that the other team realizes it will be much harder to win than they thought.
Being a ringer is a form of deception. Especially if there are bets being made on the outcome. Because of the ringer, the people betting don't understand the true odds when they bet.
It can also be used outside of a sports context. Here's an example I found:
We're at the World Bar in New York City for a speed dating event. Eight men, seven women and just eight minutes to decide if there's a love connection. But what no one knows is that today there's a ringer in their midst. Meet Lindsay. She's a professional actress.