Although seeming just as formal and strict as his wife, it is revealed in "Diaz Family Vacation" that River enjoys going off to fight monsters like Star does when his duties as king bore him. Like his daughter, River is belligerent, headstrong, and prefers battle to diplomacy. Despite this, however, he is a skilled warrior, a dedicated king, and a loving father. He comes off just as clueless of Earth customs as his daughter, and doesn't care how she uses her magic unlike her mother. But unlike Star, he has patience and planning.
In his youth, River was timid and very shy, being nervous about speaking up around others and easily unnerved when his opinions were rejected. However, he would occasionally speak out against others in defense of Moon, such as toward Count Mildrew.
As revealed in "Moon the Undaunted", River is a member of the Johansen Kingdom, a rough kingdom neighboring Butterfly Castle in the east. In the episode, River and Moon are teenagers and he sticks up for Moon when she is unsure of what to do about Toffee after he killed her mother, obviously having a crush on her. Moon appreciates the support and clearly sees his feelings for her. He even gives her an 'apology meat', a mutton leg with the word 'sorry' cut on it, saying it's how Johansens apologize. By the episode's end, River congratulates Moon for defeating Toffee and stands by her as she declares she'll scatter the Monster Army remains to protect the kingdom. Later in their lives, they get married and give birth to Star.
In "Star Comes To Earth", River appears as he should appear a royalty, but in later episodes, his true colors would show, as in "Diaz Family Vacation", he first appears talking to Star through her magic mirror with Moon stating he's going to give a lecture. Later, he catches Star disobeying her mother's orders to never leave Earth bringing Marco and his parents on a family vacation to Mewni, and as a result, he is caught having lied about his lecture. He reveals that he often sneaks out of the kingdom to fight monsters because of the exhaustion of being king, complimenting Star's habit of fighting monsters. The two promise to keep each other's secrets. In "Royal Pain", River has been thrown out of the castle by Moon for being recklessly inconsiderate and irresponsible, and goes to stay with Star in Marco's home, just as Star was having a pool party and undid it all upon his arrival because she was afraid she'd be sent to St. Olga's Reform School for Wayward Princesses if her parents found out she used magic inappropriately. But though he finds out when his beard is burnt by one of Marco's laser puppies, he actually enjoys it. Thus he and Star spend time together, messing up the Diaz' toilet, and playing golf until River destroys a castle-themed hole confusing it for the home of 'Lord Salivary of the Weebler Elf Clan'. Star is driven crazy by River's carelessness and despite not recognizing the irony of her doing the same to Marco, she acknowledges he does the same thing to Moon. River ends up deciding to stay on Earth, and invites many guests for a party until it was crashed by a sentient sun named Helios. Star drives off everyone and River acknowledges his actions and goes back to Mewni to apologize to Moon.
In "Camping Trip", River joins Star and Marco's time camping alone while Marco's parents are occupied with family paintings, treating the average camping trip in the wild as a survival trip like on Mewni. He ends up getting arrested by the rangers twice and paying fees for getting into a staring contest with a lawfully-protected bald eagle and trying to get Star and Marco a ride on an equally-law-protected bear. The two were trying to see a geyser called Old Youthful since before River arrived and he gets attached because he is insecure about his age, and he finds out not only that Marco wanted Star to see Old Youthful as a thanks for seeing things he never knew existed, but that Old Youthful's current eruption would be the last, which was actually a bluff told by the ranger sheriff for fun. River gives the two a piggyback ride to Old Youthful in 5 minutes but fails to make it in time, though falls into the geyser triggering another eruption and being stripped of his hair and burnt red.
In "Game of Flags", River and Moon have a family reunion with both the Butterfly family and River's family, the Johansens. The two families go to fallout as Star is upset she can't sit at the grownup table and instead has to sit at the kids table with her younger relatives. River declares another Game of Flags to settle the family dispute in which each family member must fight each other up a hazardous mountain and set their flag on the peak, and the winner gets to look down on the rival family members for a year. Star disobeys her mother's orders to stay at the kid's table by leaving corn dummies of her and Marco while the two compete. River confronts Star and is proud of her before Star throws him rolling down the snowy mountain. In the end, Star tries to get all the family members' flags to the top for all of them, earning her a spot on the grownup table.
In "Battle for Mewni", River throws parties all day because he is worried about Star and Moon while they are out dealing with Toffee. Marco arrives through his dimensional scissors and helps him get his confidence back and rally the people of Mewni fight off a giant monster, who confused his gestures to go away for gestures to come to him. As they celebrate their victory, Ludo arrives and takes over Butterfly Castle, locking Marco and River into the dungeon, and gives River 24 hours to have the people like him or he'll levitate him into the sky. But River refuses to leave his people when Marco frees himself with butter, at first by eating the butter, and then re-shackling himself when Marco returns with the key. By the time Ludo returns, he levitates him into the sky publicly to threaten everyone into loving him. But as Star points out, River has likely survived because he's been through worse, and is made king of large eagles that rescued the other people levitated into the sky by Ludo, returning after Toffee's defeat.
In "Scent of the Hoodie", River gives Marco a cape and offers him a chance to be a knight. But when Marco decides to live on Mewni with Star in "Lint Catcher", he finds out not only that the cape is actually his meat blanket, and that his offer to be a knight was just because he never thought he'd see him again, saying that he must be a squire first.
In "The Bogbeast of Boggabah", River forces Star to go with him on a hunting trip for the titular beast because he believes that Star needs to ease up on her impulse of rushing into things without thought when she tries to consult Moon about Eclipsa's daughter being Miss Heinous. But Star is frustrated and tries to rush through the long processes with her wand and not thinking about the consequences, resulting in them starting the entire thing over, and ending with River throwing out his back and deciding to rest and start over again in the morning. Unbelievably frustrated, Star tries to take down the Bogbeast herself but gets stuck in a mudpit. Just as she thinks she's doomed, River and the bog inhabitants including his friend Eddie name her the Bogbeast, which is actually not real, and the hunt is just a game where someone dresses up as the Bogbeast and gets chased through the woods. River tells Star that her impulsiveness and tendency to rush into things without thinking may be her specialty, but they are also good traits of a Bogbeast. He hopes she learns from this hunt not to be so spontaneous and stand in her own way of her goals. As the crowd of Mewmans prepares to have a bog party, King River says he must keep his promise to Star and return her to the castle, but Star decides to join the party anyway.
In "Conquer", he and his giant eagles try and fail to defeat a rampaging Meteora.
In "Butterfly Follies" and "Escape from the Pie Folk", River has to deal with the Pie Folk, renowned con artists from Pie Island, to find a lost Moon, who lost her memory because of her time in the Realm of Magic. Moon regains her memories and the two decide to leave together now that they aren't leading anymore. In "Down by the River", they create a town for Mewmans who don't like Eclipsa's rule.
In "Cornonation", River is seen chasing down a fleeing Globgor until Star comes and stops the chase. They return to Eclipsa's coronation and River supports Globgor for only being a monster to protect his family, forming a companionship with him as a brave father. This companionship shines in the season's climax when he and Eddie vow to escort Globgor, who has been wounded by a Solarian blade that nullifies monster powers and eventually disintegrates them upon the wounds' growth, to the Sanctuary. But they are stopped by Mina Loveberry before they can make it.
Role in the series