Frank Cosgrove Jr. is a supporting character in the Netflix series, Ozark. He is the son of the head of the Kansas City Mob.
Frank hangs out most of the day at the casino and pissing off Ruth. He works for his father and arranges deliveries of cash with Ruth; their increasingly antagonistic interactions lead to escalating actions against one another. His hot-headed temperament leads to a variety of issues for the Byrdes, Ruth, and his father Frank Cosgrove.
Frank forces his way on to a high-stakes poker game Ruth is hosting at the Missouri Belle and proceeds to cause a scene, disrupting the game and getting thrown out. He begins loan-sharking at the Belle until Marty puts a stop to it.
Frank gets drunk after his men are killed and goes to the Missouri Belle to confront Ruth and almost blows their cover; Ruth forces him to leave. He waits for Ruth to be the last one out and viciously beats and almost kills her. After the Byrdes refuse to enact retribution on him after his assualt on Ruth, Darlene Snell takes it upon herself to shoot off his genitals with a shotgun in an effort to get Ruth on her side.
In Season 4, Darlene approaches Frank Jr. after his father won't do business with her. He agrees to buy some of her product. Later, Ruth tells Frank she needs the product back and offers to buy it for an extra fee. At the end of the season, Ruth tells Frank Jr. the truth about what happened to his father -that Darlene shot him. She tells him to do what he needs to do, but eventually takes it back because Wyatt loves Darlene. After seeing that Darlene was shot, Ruth confronts Frank Jr. about it but he says he didn't do anything.
Frank starts to form a pseudo-friendship with Ruth. He gives her a gun when she says she needs to kill someone (Javi) and tells her he'll buy her a drink when it's done. When Ruth wants to make a deal with Clare Shaw, Frank helps her move the product from Darlene's house in exchange for a fee.