After his mother gets a new job in Houston, he and his father Guy move there, along with Diane a while later. Diane marries Guy shortly before the events of the finale, becoming Sonny’s step-mother in the process.
Sonny is a teenage male bison with dark brown fur and horns on top of his head who resembles his father Guy. He has acne.
He wears an unbuttoned gold and navy Varsity lacrosse jacket, black shirt, blue jeans with a hole in the left leg, and grey sneakers.
Sonny is a teenage boy, reluctant to accept his father dating new women, telling Diane that his parents are going to get back together. However, when she says they hate each other, he breaks down crying showing how vulnerable and sensitive he is to subjects that concern his parents.
He plays Varsity lacrosse and often tries his best to look sporty and macho, acting dismissive towards "girly" things. This is shown when he hides the fact he has been reading Diane's advanced readers' copy from his friends in The Horny Unicorn.
However, he shows an interest in Diane's Ivy Tran book series, relating to its content despite his attempts to downplay it and maintain his image as a cool jock. He warms up to Diane after talking about her book.
Sonny was born to Guy and Lady, presumably sometime in the mid-2000s. His mother and father divorced which is an event that he holds resentment over. He refused to speak to his dad when he tried calling him in Feel-Good Story. This may imply Guy was the one to call off the marriage.
In Xerox of a Xerox, Diane and Guy are having lunch at Parmadillo's and Guy notices Diane isn't listening to what he's saying and watching the news segment with BoJack and Biscuits. Guy then tells her if she wants to watch it's OK since BoJack is her friend and this is a really big deal for him.
He goes on to say Diane doesn't really talk about herself or her past with him. Diane says she doesn't want to get dragged back into that other stuff. Diane then retorts she has been living in Chicago for six months and she still hasn't met Guy's son. Guy then tells her she can meet Sonny tomorrow.
At Parmadillos, Diane is finally introduced to Sonny by Guy. She tells him it's nice to finally meet him. Guy and Diane congratulate Sonny for making the Varsity lacrosse team. Guy then leaves to get cheese fries and while he's gone Sonny asks Diane what her deal is. He goes on to say his father has a thing for broken women and they leave after he nurses them back to health.
Diane then asks if his father thinks she's going to leave him. Sonny responds it doesn't matter because his parents are still in love and going to get back together. Diane says that his parents hate each other and they fight all the time. Sonny then bursts into tears and Diane detracts her statement about his parents hating each other. Guy arrives back with their food and says he was gone for two minutes.
In The Horny Unicorn, Sonny and his friends are playing video games in Diane and Guy's apartment. Diane comes in and asks where the advanced readers' copy of her book is. Sonny tells her he would never read it as he feels like it's a book for girl losers, which his friends laugh at. Diane then tells him to let her know if he's seen it as she needs to get the revisions to her publisher. Sonny retorts that sounds like a "Diane" problem before she walks off.
Later, Diane is playing video games when Sonny walks in. He hands her the advanced readers' copy of her book. Diane asks Sonny if he read it and he tells her he has been reading it. He then tells her he wasn't going to say so in front of his friends but he finds the story to be unrealistic. Diane questions what he means.
He tells her the character Ivy Tran lives in Schaumberg, which is not in Chicago. Diane tells him it's not too late to change it. Sonny then asks her questions about the book and if those situations are real and Diane confirms they are. He then tells her he thinks it'll be a good book for girls with low self-esteem. Sonny then asks her when the next one is coming out and she tells him if she gets started right away it should be out by next fall. He then gives her his encouragement.