Rionach Steward (born c. 1634) was a half-blood witch, and the younger twin daughter of Isolt Sayre and James Steward, who were the founders of Ilvermorny School of Witchcraft and Wizardry. Rionach Steward would later become one of the school's first Defence Against the Dark Arts instructors, teaching there for many years.
Rionach Steward was born in or around 1634 at Ilvermorny in Massachusetts, along with her older fraternal twin sister, Martha. Her parents, Isolt Sayre, a pure-blood witch, and James Steward, a No-Maj, were the founders of the school. She also had two older brothers by adoption: Chadwick and Webster Boot. She was named after her late maternal grandmother.
Rionach was born with magic powers, whereas her elder twin was born a Squib. It was also rumoured — though it was never confirmed by the family — that she was born a Parselmouth, a frequent trait in her maternal family, descendants of Salazar Slytherin (sometimes known to his contemporaries as "Serpent-tongue"), and which her mother herself seemed to possess.
When she was a baby, her great-aunt, Gormlaith Gaunt, attacked the family due to her prejudice against Muggles. Her parents had been put in a deep sleep by Gormlaith, but the cries of baby Rionach and her sister Martha woke them, since the curse took no account of the power of love. The family were saved by William, the Pukwudgie her mother had befriended several years earlier.
Rionach went on to become the teacher of Defence Against the Dark Arts at Ilvermorny. She never married or had children, determined not to let the Slytherin and Gaunt bloodlines to continue (which was ultimately fruitless, since the American branch of the family was unaware that Gormlaith was not the last of the Gaunts in England).
- Pottermore (First mentioned)