Taylor Mason, a non-binary main character, very much breaks the mould of an otherwise typical drama show, and is played by a non-binary actor named Asia Kate Dillon. Initially they are an anti-social stereotype of super-intelligence, but develop an actual personality throughout later episodes, including romantic relationships with both men and women.
Lauren, Taylor's coworker, about to kiss them for the first time in episode S04E09.
Their adversaries, even when plotting how to take them down, continue to use their correct pronouns. The consensus is generally that Taylor’s business practices and dealings are fair game to criticise, but their gender is not. I really enjoy that aspect of the show, and the fact that Taylor is portrayed as an clever, capable person who isn't afraid of conflict.
They are misgendered a few times by antagonistic and ignorant characters, and also by their father, who is struggling to accept their gender. The misgendering gets less common throughout the show, and with every season Taylor's gender is less of a topic to be actively discussed. Billions is probably the most long-running example of normalising gender-neutral pronouns in everyday use.
The show is ongoing and Taylor remains an influential main character. But there’s no escaping the fact that Billions' central themes include capitalist corruption and rich people’s drama. Taylor themself plays "the good millionaire" in later seasons, which– given the current dialogue about wealth inequality in America– may dissuade some people from watching for the non-binary representation.
The worst aspect of the show is its misogyny, including the tokenising and manipulation of a female rape survivor named Martina Slovis. Season 1 kicks things off with nameless women in a prostituting massage parlour and a nameless dwarven woman offering sexual services in a bathhouse, so that should give you an idea of the show’s standards when it comes to women and minorities. Even in later seasons, when the show is apparently more progressive, the m-word is used by a character recommending dwarf-tossing as an activity to boost office morale. This is a reference to The Wolf of Wall Street that's pretty tasteless, considering modern awareness of the damage such a practice causes. A cis gay character also jokes about a colleague supposedly having a vagina, meaning he's a weak and pathetic man. So yes, there's a non-binary main character, but there's also anti-FTM transphobia.
I would recommend reading reviews of the shows/episodes before you dive in, especially considering racist storylines in recent seasons (one of which involves Taylor directly), and a poorly-handled storyline about police brutality against a man of colour. Taylor is an example of good non-binary representation in the sense that they're played by a non-binary person and allowed a personality beyond their gender, but they're not a very good person. Pretty much every main character is morally corrupt in some way.
Chuck and his father.
One of the creepiest aspects of this show, in my opinion, is the relationship between Chuck (a main character) and his abusive father. At 14 years old, his father and uncle hired an adult female prostitute to have sex with him and, as an adult, Chuck's father kissed his son full on the mouth and held him still even as Chuck tried to resist him. Not only did Chuck's wife react with mere mild surprise at her husband's description of statutory rape, the show does not treat his experiences seriously. It's a repugnant storyline that almost kept me from watching, especially when you consider how a character might react if a 14 year-old girl had been coerced into sex by her father and uncle.
In another scene, a minor character minimised the seriousness of an adult female teacher having sex with an underage male student, because the student would surely want it. The show doesn't treat sexual assault well in any respect, but has a particular trend of mocking male survivors.