Chelsea is a princess from a magical world who arrives on Earth determined to find a husband. She secretly possesses magical girl powers, a truth she must hide from humans. This secrecy shatters when Kazuya accidentally discovers her true nature during their first meeting. Bound by the traditions of her homeland, she interprets this accidental exposure as fate, decreeing Kazuya her destined husband. She promptly moves into his residence, joining the other girls living there whose secrets were similarly exposed.

Chelsea displays an intense fixation on Kazuya, often lost in elaborate fantasies about their future life together. She harbors deep resentment towards the enigmatic Guardian Heart, blaming this figure for constantly thwarting her attempts to grow closer to Kazuya. Unaware that her romantic rival, Hina—another resident of Kazuya's house—is actually the Guardian Heart in disguise, Chelsea directs persistent frustration and rivalry towards her.

This antagonism towards the Guardian Heart drives much of Chelsea's activity. Within the household group, which includes figures like Maya the ninja and Kurusu the alien with cosplay powers, Chelsea sometimes forms temporary alliances. These collaborations, often with girls sharing her grudge against the Guardian Heart, focus on schemes to expose or confront their mutual adversary. Typical plans involve manipulating situations where Kazuya is alone with another girl, hoping to draw out the Guardian Heart. Throughout these efforts, Chelsea remains oblivious to Hina's double identity. Her role centers on pursuing Kazuya romantically and opposing the Guardian Heart's perceived interference.