Captain Peanutbutter is an adult 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. Perhaps 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 like 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 being 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—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, and 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 OK. 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 and 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.
- He has two unnamed sons but their mother is never shown.