In *Sasurai no Shōjo Nell*, the figure covertly aiding Nell and her grandfather Trent during their flight from the loan shark Quilp is identified as the "Mysterious Protector". This protector is later revealed to be Mario, Nell's estranged older brother. Their separation stems from family circumstances: after Nell's father died during her infancy, her paternal grandfather Trent requested custody of her, while her mother Rosa retained Mario. Consequently, Nell and Mario were raised separately, unaware of each other's existence.

Mario discreetly safeguards Nell and Trent throughout their journey, intervening to help them evade Quilp's pursuit while initially concealing his identity and motives. The narrative clarifies that Mario learned of Nell's plight and chose to protect her anonymously. His protective role culminates in a family reunion where Mario, Nell, Trent, and Rosa reconcile in Ireland—referred to as "Paradise" in the series. This resolution differs from the original Charles Dickens novel. The character of Mario as Nell's brother is an original creation for this adaptation, drawing inspiration from a similar narrative device used in an earlier TV Tokyo production, *Isabelle of Paris*.