Satsuki Myoudouin is the older brother of Itsuki Myoudouin (Cure Sunshine). Afflicted by an unspecified chronic illness limiting his mobility, he relies on a wheelchair and avoids strenuous physical activity. This condition prevents him from inheriting the Myoudouin family martial arts school, a role tradition reserves for male heirs. Consequently, Itsuki cut her hair and cross-dressed as a boy to become the heir in his stead.

Satsuki possesses a warm, compassionate nature and deeply worries about Itsuki's well-being. He fears her dedication to upholding the family tradition forces her to neglect her personal life and suppress her interest in feminine pursuits like fashion. Despite his health challenges, he maintains optimism and inner strength, rarely voicing complaints. His Heart Flower is chamomile, symbolizing resilience through hardship.

A pivotal moment occurs when Satsuki faces surgery for his condition. He initially hides his fear of the operation and its implications, not wishing to burden Itsuki after her sacrifices. This fear is exploited by the Desert Apostle Sasorina, who transforms his wheelchair into a Desertrian. Witnessing his vulnerability and the attack, Itsuki unlocks her powers as Cure Sunshine to save him. Following the Desertrian's defeat, Satsuki undergoes successful surgery. Later, demonstrating his recovered mobility, he engages Itsuki in a martial arts match and defeats her, proving his capability and relieving her of the heir's burden.

Satsuki continues as a supportive figure in Itsuki's life, actively encouraging her to embrace her love for cute, girly things, including joining the school's fashion club. His journey and relationship with Itsuki are symbolically referenced during Cure Sunshine's trial in the Pretty Cure Palace. The trial presents the false accusation that pursuing her interests means abandoning martial arts or her brother. Itsuki rejects this, affirming that embracing her true self—including fashion—honors Satsuki's wish for her happiness.