Wendy and Marty referred to Ben in earlier seasons of Ozark, but he never appeared on the show until Season 3. He was working as a substitute teacher when he lost his temper when one of his students had a compromising picture sent to the whole class. He collected all his students phones, threw them into a wood chipper, and then assaulted a nearby worker, subsequently getting fired from his job.
Running from the police, he goes to find his sister in the Ozarks. He shows up unexpectedly, leaving them to assume that he was in a bit of trouble. Ben asks to stay, which at first they are hesitant about. Marty directly asks Ben to leave but Wendy eventually tells him to stay because she needs the support.
Ben helps out around the house and hangs out a lot with Jonah. He gets a crush on Ruth Langmore and asks her out several times, even though she continues to push him away. Despite this, Ben and Ruth develop a relationship and eventually fall in love, even though Ruth knows that Ben has bipolar disorder and went off his meds. Wendy pleaded with Ruth to break things off with Ben because when he's off his meds he's a danger to himself and others, but Ruth continues to see him anyway.
Ben starts to blow up at people- at Wendy, at Marty, and at Helen. He tells Erin, Helen's daughter, that her mother is a lawyer for Omar Navarro, which triggers Helen to give the order to have Ben killed— for the purpose of not allowing Ben to reveal anything about the operation.
Ben tries to go on the run - first with Ruth and second with Wendy, but he proves to be an unreliable danger after multiple outbursts; after calling Helen in an attempt to "fix things," Wendy gives up. She calls Helen to tell her where Ben is and the cartel hitman, Nelson, is sent to get Ben. Nelson brings his body back to be cremated in Marty's funeral home.
Ben is, in many words, complicated. He's a good hearted man who wants to do the right thing, though having bipolar means that he's extremely erratic and unpredictable, often having violent outbursts and oscillating mood swings when off his medication that he takes for his illness. His intentions are usually good, but his illness hijacks those intentions and often makes him act in a way that he usually wouldn't.
Ben shows himself to be loyal to the people he cares about, particularly Ruth and Wendy, though his relationship with Wendy is strained due to their troubled childhood and Ben's illness. Ben is resentful that Wendy seems to look down on him because of his illness, but he nonetheless loves her and would do anything for her. He also forms a bond with Jonah, which seems to fill the void for Jonah having a best friend after Buddy's death, with Jonah taking his death the hardest asides from Ruth and Wendy.
Ben is very impulsive, usually not considering the consequences of his actions. This is exemplified by when he decides to get rid of his medication, punching Marty, angrily telling Helen's daughter what she really does, calling the cops about his situation, and calling Helen in an attempt to apologize. It is this which causes him to become a liability.
Once he learns the extent of what Wendy and Marty really do and he gets off of his medication, Ben quickly spirals out of control and begins acting out, this includes punching out Marty, angrily revealing to Helen's daughter what her mother really does for a living, and cursing out Marty and Wendy for institutionalizing him again. His episode ends up accelerating his rage and resentful feelings towards Wendy, as well as his anger over getting institutionalized again.
Unfortunately, it was Ben's erratic and impulsive behavior that ended up getting him killed, and his death caused Ruth's relationship to Marty and Wendy to be severed entirely, as well as Jonah's relationship with Wendy to be permanently damaged, likely beyond repair.