Horizon Ariadust (ホライゾン•アリアダスト, Horaizon Ariadasuto) was a member of the Matsudaira family and the childhood friend of Toori Aoi. She was killed in an accident in the year 1638 TE, and a marker has been erected in Remorse Way, commemorating her death. Her death is considered to be among the most influential events in Musashi, and is known by many people inside and outside Musashi.
- Horizon's surname is created by taking her original surname of Matsudaira and omitting the first syllable, then reversing the letters. MATSUDAIRA → TSUDAIRA → ARIADUST.