Milro is princess of the Wonder Planet's Waterdrop Kingdom, daughter of King Pump and Queen Yamul, and older sister to the infant Prince Narlo. Reserved and kind towards her friends and subjects, she possesses a strong sense of duty, undertaking tasks for her kingdom even when reluctant. Her mother's strict expectations regarding royal behavior make expressing her own desires difficult.

Milro passionately aspires to be an artist. Initially creating art based solely on her mother's directives, encouragement from Princesses Fine and Rein of the Sunny Kingdom gives her the courage to paint subjects of her own choosing. This breakthrough fosters broader confidence to voice her thoughts and act independently.

Her portrayal diverges between continuities. In the anime, Milro builds confidence gradually, notably securing victory in a Sports-themed Princess Party to benefit her kingdom. The manga depicts her exhibiting confidence and a sense of superiority, believing herself the planet's most beautiful princess and often causing trouble for Fine and Rein. In this version, she wields the Drop Wand, granting the power to freeze objects, yet uniquely never wins a Princess Party.

Milro takes a caring, responsible role with her baby brother Narlo. Narlo, in turn, shows a fondness for Princess Milky of the Moon Kingdom. Her attire consistently features light blue with water-drop motifs and a red gem, typically including a blue hat and a white tiara also bearing a red gem.