Sanae Dekomori is a third-year junior high school student who later advances to high school. She first met Rikka Takanashi online two years prior through discussions about parallel worlds, leading her to become Rikka's self-proclaimed servant after recognizing Rikka as the "Tyrant’s Eye" from a "previous life". Like Rikka, she embraces chūnibyō, adopting the persona "Mjolnir Hammer" and wielding her long, blonde twin-tails—weighted with bags she calls "Mighty Mjolnir Maul"—as weapons, though they frequently cause her to trip or become entangled. She later "switches" to using a giant hammer in her delusional battles. Despite her chūnibyō behavior, she excels academically, particularly in mathematics, having completed the middle school curriculum and solving high school-level problems with ease, making her popular among classmates. She maintains normal behavior around peers outside her chūnibyō circle and is aware her delusions are fictional. She comes from a wealthy family, occasionally arriving at events in an upper-class sedan accompanied by a butler. Her speech is characterized by ending sentences with "-desu" or "-desu ka", deliberately pronounced to resemble the English word "death" (e.g., "De-de-death!!"). She dislikes milk and dairy products but consumes them daily in hopes of growing taller, though she remains petite at 143 cm (4'8"). She is also prone to motion sickness. She obsessively seeks the real "Mori Summer" (a delusional identity from Shinka Nibutani's past), dismissing Shinka as "Fake Mori Summer" due to Shinka's rejection of her chūnibyō history, fueling their antagonistic relationship. Sanae owns multiple copies of Shinka’s chūnibyō-era spellbook, the "Mabinogion". Their dynamic evolves: an accidental kiss occurs in an OVA after they fall, which Sanae blushes over and denies enjoying. Later, during a shared bed scenario in the *Take On Me* movie, she initially draws a boundary line but ends up hugging Shinka in her sleep. In *Season 2*, as a high school freshman and class representative, she surpasses Shinka’s entrance exam score and garners admiration for her intelligence. By the *Take On Me* movie, she serves as student council president. She and Shinka are coerced by Touka to pursue Rikka and Yuuta under threat of exposing their kissing photo, endangering Sanae’s presidential reputation.