If there was a contest for the most mother figures one child can have in their life, Myu would surely be a finalist. In Arifureta: From Commonplace to World's Greatest, an isekai harem anime, viewers are introduced to merfolk toddler Myu towards the end of the first season. The tough guy protagonist Hajime Nagumo and big-hearted bunny girl Shea Haulia find the adorable merfolk child first. Overtime, Myu goes from calling Hajime "Big Brother" to "Papa," much to his dismay.
There are many questions left unanswered in the series, but the story of how Myu became a daughter to Hajime and Yue is a well-depicted development. The gang first encounters sweet little Myu in Episode 11. While on a reluctantly promised date with Shea, Hajime detects the presence of a weak child in the sewers below using his sense presence skill. They find her unconscious, but still breathing. When she regains consciousness, she explains to Shea that she was taken from her mother. She'd been escaping slave traders. Hajime and Shea agree on leaving her to the city guard.
As they are walking away, they hear an explosion. They discover a ransom note written in the wall, which claims that if Hajime doesn't deliver Shea to the kidnappers, they'll kill Myu. Hajime makes it his mission to save the child. He recruits the help of Yue and Tio Klarus, and they eventually find the antagonists' hideout. They realize they're up against slave traders who are running a monster child trafficking ring. As they disband the evil operation, Hajime eventually finds Myu with the kingpin, trapped in a glass orb filled with water. Without hesitation, Hajime shoots the kingpin dead and saves Myu. Before being reunited by the rest of the gang, Hajime shows her a "fireworks display" by enthusiastically setting off explosives in the slave trader hideout.
When everyone reunites, Hajime voices his concerns about being called "Big Brother." Myu gleefully retorts by saying she wants to call him "Papa." She exclaims that since she doesn't actually have a father, she can promote her big brother instead. Witnessing this adorable exchange, Yue tells Hajime she wants a baby with him. Tio and Shea immediately follow suit. True to this anime's harem classification, Myu's debut episode ends with everyone fighting over Hajime.
In the remaining two episodes of the season, viewers can see Hajime warm up to his "daughter," referring to her as such and being very protective of her regarding any perceivable threat. When Kousuke Endou -- a classmate of Hajime's from his previous life -- snaps at Myu, Hajime rightfully snaps back, telling him to not talk to his daughter like that. Yue notices this, pointing out that Hajime really did become a "Papa" to little Myu. She is the only one besides Yue to have truly broken through the hard exterior Hajime built around himself while in the labyrinth. Although her real mother is still out there somewhere, it will surely will be hard for Hajime and Yue to part ways with Myu now that they've found each other.