The story begins with the protagonist, who was a 29-year-old Japanese woman named Erena in her previous life, awakening as a baby named Elenika Arrent in a different world. Her earliest memory is of her father, the Duke, sternly telling her brother, Theodore Arrent, that his entire existence must be for Elenika. Elenika quickly realizes she has been reincarnated into an otome game called "Chamomile's Light," and worse, she is the villainess, Elenika Arrent.
As a baby, Elenika spends her days sleeping and eating, but she is aware of her surroundings. She learns that her parents are a Duke and a beautiful former "Fairy Princess," and that Theodore, the Duke's adopted son, is her older brother. Theodore was found as a three-year-old in the slums and brought into the Duke's household, initially experiencing a dream-like life. However, after Elenika's birth, the Duke's attitude towards Theodore drastically changes. Theodore is treated coldly, receiving only the bare minimum of necessities and undergoing rigorous training to be the heir, while also enduring physical punishment. The Duke's nightly ritual of forcing Theodore to swear his life is for Elenika deeply traumatizes him. Elenika's mother, overjoyed that her husband's affection is solely for their daughter, becomes indifferent to Theodore's plight. Theodore grows up as an emotionally stunted "doll" destined to exist only for Elenika.
By the time Elenika is three, she has fully grasped her situation. She understands that in the game, the villainess Elenika Arrent meets a terrible end, often being killed by Theodore, or exiled, or sent to a convent. This is largely due to Elenika's own selfish and arrogant behavior, fueled by her parents' indulgence and her mistreatment of Theodore. Elenika decides to avert this fate by implementing a "Brother Complex Strategy" (Burakon Sakusen). Her goal is to prevent Theodore from becoming a resentful, emotionless individual and to avoid her own demise.
Elenika's strategy involves spending as much time with Theodore as possible. She insists on sharing meals with him and even requests to sleep in his room, which leads to their rooms being moved next to each other. This intervention, coupled with Elenika's constant affection, begins to soften Theodore. He starts to smile more, especially around Elenika, and becomes less fearful of his father.
By the time Elenika is six, she is actively trying to make Theodore happy. She uses her "cute pleading face" to convince him to take her on an outing to the city, which he successfully obtains permission for from their father. This outing marks their first time outside the estate. During the outing, Elenika witnesses Theodore being protective of her and realizes the depth of his care.
At age seven, during a royal birthday party, Elenika gets lost and encounters a group of noble boys bullying a younger boy, who is revealed to be the Third Prince. Elenika intervenes, throwing her hairpin at the bullies. Theodore arrives and fiercely defends Elenika, scaring off the bullies. He then takes the Third Prince under his care, and Elenika, despite her initial intention to return to the party, ends up leaving with Theodore and the Third Prince. This incident leads to a conversation where Theodore explains his father's decree that he is responsible for Elenika, and that the noble boys' families will face consequences for their actions.
By age thirteen, Theodore is preparing to enter an academy. Elenika continues her "Brother Complex Strategy," ensuring Theodore prioritizes her. She buys him a necklace as an entrance gift, hoping it will encourage him to take care of himself. While shopping, Elenika again intervenes when she witnesses a noble boy bullying a younger girl. She uses her status as a member of the Arrent Dukedom to deter the bully. Theodore, upon finding Elenika, is relieved she is safe and expresses his concern, reinforcing his protective role. The chapter ends with Elenika determined to continue her efforts to ensure Theodore's well-being and avoid her own tragic fate.
