Captain Peanutbutter is an adult male Labrador Retriever. He is Mr. Peanutbutter's age, in his early or mid-fifties.
He shares a little resemblance to his brother, Mr. Peanutbutter; but he has a fat belly.
He has a slightly darker fur in a golden yellow color, a fluffier neck, slightly thicker eyebrows, wrinkles under his eyes, and a deep raspberry nose.
He wears a plaid red and black lumberjack shirt, blue jeans with brown suspenders with metal buckles, dark yellow lace-up work boots, and a blue-grey baseball cap.
Captain Peanutbutter is just as cheerful and friendly as Mr. Peanutbutter—but seems to be the more mature in demeanor, out of the two brothers. This may be a result of being five minutes older than Mr. Peanutbutter.
He shows signs of depression at first, which turns out to be a result of his twisted spleen. He appears to be much better after having the spleen removed.
He was very accepting and welcoming, of his ex-sister-in-law Diane Nguyen, and treated her as part of the family.
Captain also seems to be a caring father to his two sons. He worries about what will happen to them if he dies—as a result of his spleen and asks Mr. Peanutbutter to take care of them if something happens to him.
Captain Peanutbutter was born five minutes before Mr. Peanutbutter. He grew up with him on the Labrador Peninsula and has two unnamed sons.
In Old Acquaintance, on New Year's Eve 2015, Mr. Peanutbutter and Diane fly to the Labrador Peninsula to spends New Years with Captain Peanutbutter—who picks them up from the airport.
The two brothers are excited to see each other. However, whenever the two are alone Captain Peanutbutter spouts nihilistic and depressing thoughts to Diane about life and death.
She worries he knows about her abortion, and she keeps telling Mr. Peanutbutter to talk to his brother. However, Mr. Peanutbutter cheerfully denies there is a problem and assures Diane he's probably just joking with her.
When they are in bed at night, after Diane tells him to talk to his brother again, Mr. Peanutbutter assures her that if there was a problem his brother would tell him.
When Diane tries to convince him again, Mr. Peanutbutter gets angry at her, questioning if she thinks she knows his brother better than him. He says just because she has a bad family, doesn't mean everyone else does. He quickly apologizes and tells her he has to go for a walk.
Mr. Peanutbutter goes outside and asks Captain Peanutbutter if everything is OK with him. Captain Peanutbutter reveals that he has a twisted spleen, he needs an operation, but it's not fatal and everything's going to be fine.
Mr. Peanutbutter is upset by this news and asks Captain Peanutbutter if he's sure everything will be okay. Captain Peanutbutter tells him one day he'll have to be "The Captain." An emotional Mr. Peanutbutter jokes about how depressing their conversation got, and they laugh and howl at the moon.
Mr. Peanutbutter becomes depressed over this, as seen in It's You, along with saying nihilistic remarks. However, he is asked to announce the Oscar nominees, but he turns it down because of his emotional state.
When Mr. Peanutbutter tells this to Captain Peanutbutter over the phone, Captain Peanutbutter tells him to not miss this opportunity—he promises his brother, he'll call him after his surgery.
Later, at the Oscars, Mr. Peanutbutter and Todd accidentally drop his phone after it rings, making them leave the area to find it. After doing so, it is revealed it was Captain Peanutbutter calling, and he announces his surgery was a success.
Captain Peanutbutter attends the surprise wedding for Pickles and Mr. Peanutbutter in Surprise! with his wife. At Mr. Peanutbutter and Pickles' house, BoJack introduces Eduardo, the sober companion, to Mrs. Captain Peanutbutter and Captain Peanutbutter. He explains Eduardo is here to keep him out of trouble. Captain Peanutbutter asks if Eduardo is one of those fainting goats. Eduardo laughs and says only if he panics.
BoJack interjects that nothing makes Eduardo panic, because he is a trained sober companion. Captain Peanutbutter then starts talking about what makes him panic. He then says he is most of the time, he is an ocean-levels-rising guy. However, sometimes he likes to dwell on the fact, that there will always be invisible walls between himself, and those who claim to know him.
Eduardo admits that is pretty nerve-racking, but as for himself, the only thing that makes him panic is the thought of him drinking again. Captain Peanutbutter raises a toast, to that never happening again, with a bottle of alcohol. Eduardo faints and BoJack sarcastically says that's very helpful. BoJack then thanks Eduardo.
Pickles says she is telling everyone about Mr. Peantbutter's infidelity because she's a Gemini. Mr. Peanutbutter then says to think of his family, as they need him to be strong right now. Mr. Peanutbutter goes on to say Captain Peanutbutter feels like he is trapped in a loveless marriage with his wife. Captain looks at Mrs. Captain Peanutbutter guiltily. Pickles then asks who could really blame her. Pickles say every time she gets into a conversation with Captain she thinks "If I were married to this guy I'd gnaw my own hand off just to get the ring off my body."
Later, various guests pop out from their hiding places, and there is a universal sigh of relief. Everyone then makes a mad dash for the door, and then they subsequently leave.
- Captain has two unnamed sons.