At the start of the series, Buff Frog was of Ludo's minions, whose main role was to spy on Star Butterfly during her stay on Earth in order to inform Ludo of the best chances to steal her wand.
However, after Ludo hires a new monster named Toffee into his army, Buff Frog grows increasingly suspicious of him, especially after he steals his position as spy by replacing him with remote-controlled surveillance camera. After Toffee formulates a plan to steal the wand during Star's reenactment of a Mewnipendence Day, Buff Frog volunteers to be the one to carry it out. However, the mission fails after the artifact that Toffee gave to Buff Frog to transport the wand to Ludo malfunctions and gets his cover blown by trying to steal it with his hands. Despite Buff Frog insisting that Toffee sabotaged him to humiliate him, Toffee uses the mission's failure to convince Ludo to fire Buff Frog and kick him out of the castle.
During the season one finale, after being betrayed and usurped of his position of leader by Toffee, Ludo tries to convince Buff Frog to help him reclaim the castle but the monster turns him away do to still being angry for being fired. However, Ludo gets to his soft side by giving him a bunch of tadpoles as a gift, whom Buff Frog adopts as his children. Little after, Star arrives and questions them about Marco's whereabouts, who was kidnapped by Toffee's minions. Ludo ditches Buff Frog in favor of Star to assult the castle, but the princess ditches the little villain and instead follows Buff Frog inside, showing more trust to him after he cut his ties with Ludo. Together, they confront Toffee and his minions but get overpowered and then, Toffee forces Star to destroy her wand in order to save Marco's life. As the wand seemingly perishes, Star warns Buff Frog to run. Buff Frog quickly leaves the place moments before the wand explodes and destroys the whole castle.
After becoming a parent, Buff Frog shows great love and care to his adoptive children, wanting only the best for them. He demonstrates more compassion than before, as he is unwilling to torture others for information. He has also discarded all loyalty towards Ludo; when Ludo attempted to recruit him again, Buff Frog punched him in the face, believing Ludo had lost his mind. This is further shown when he sends letters to Star to warn her about Ludo and gives her a map on Ludo's old hideout, but because his handwriting is illegible, by "Raid the Cave", Star only learns of his messages by visiting him.
By "Puddle Defender", Buff Frog is very good friends with Star, offering her and her mother a place to stay while they are on the run from Toffee. In an effort to prove to Queen Moon that not all monsters are bloodthirsty and vicious, he sides against Star and keeps her locked inside his house, then goes with Moon to save her in "Toffee" and helping stop Toffee.
In "Starfari", after helping stop the monster village from being destroyed by a flood, Star names him the new monster expert for Butterfly Castle. However, during "Is Another Mystery", he turns up missing and Star goes with Tom to look for him, only to learn that he, his children, and several other monsters are leaving Mewni for another dimension after giving up hope for a peaceful coexistence with Mewmans, promising Star that he'll keep in touch and they might return someday.
In "Cornball", Star brings Buff Frog to a cornball gaming event, a Mewnian game, in which both Mewmans and monsters compete, as a guest for Eclipsa, with Kelly and Marco as the commentary, and hopefully convince him to come back to Mewni, as Star found him through Katrina, who's now fully developed and pre-adolescent along with the other tadpoles. But things go awry when Mewmans and monsters still keep fighting starting with the competitors splitting in their teams by race, convincing Buff Frog it was a mistake to come. Though they eventually find out that Mewman and monster children are playing a cornball game together outside the stadium. This convinces Buff Frog to stay for hope that the next generation of Mewni will turn out better.
Role in the series
The new dimension Buff Frog and the other monsters live in is dubbed New M'onstrarus, and is ruled by Taffee, whom Buff Frog does know is Toffee's cousin, albeit hasn't become as insane as Toffee nor Coffee, however not anymore willing to trust her bloodline due to the tainted reputation her clan now bares, but she was still successful in convincing him to go along with hundreds of other monsters to leave Mewni in being worried that not all Mewmans would be like Star. Thus he and his children continue to live in her dimension.