Cosmo Hōjō is the older sister of the idol Sophie Hōjō, a member of the main unit SoLaMi Smile. As their parents travel overseas for work, Cosmo has taken on the role of Sophie's primary guardian, caring for her at home and looking after her well-being.

In terms of appearance, Cosmo is a young woman with bright turquoise or blue-green hair that falls to her neck, with a style that often covers one of her purple eyes. She typically wears a space-themed dress and planet-shaped earrings that complement her cosmic-inspired name. Laala Manaka, the series protagonist, has remarked that Cosmo is very pretty.

Cosmo works as a fashion designer within the world of PriPara, contributing her creative talents to the idol world behind the scenes. This profession aligns with the series' focus on dress-up and coordinate creation, though she is not typically seen performing as an idol herself. Her personality appears friendly and kind, and she is known for her characteristic catchphrase of expressing gratitude with "Mega-Cosmic Thanks".

Her primary function in the story revolves around her relationship with her younger sister Sophie. Cosmo serves as a supportive presence, helping to maintain the household and providing stability for Sophie as she pursues her idol career. While Sophie is famous in PriPara for her sophisticated Fancy Mode persona, Cosmo is aware of her sister's true nature, including the moments when Sophie's energy depletes and she becomes dazed and helpless without pickled plums to recharge.

Cosmo does not undergo significant character development arcs herself, instead remaining a consistent support figure. Her notable abilities include her fashion design skills, which contribute to the creative aspects of PriPara, and her capacity to manage household responsibilities while serving as her younger sister's guardian.