- "There was a girl there, about his own height and perhaps a few years older than him. She had silver hair, and eyes with a pearly sheen to them, and the kind of interesting but impractical long dress that tends to be worn by tragic heroines who clasp single roses to their bosom while gazing soulfully at the moon."
- —Mort upon seeing Ysabell.[src]
Duchess Ysabell of Sto Helit was the adoptive daughter of Death, wife of Duke Mortimer and mother of Susan Sto Helit. Her birth parents were killed while she was a child, so Death took her to his domain and raised her as his own.
Ysabell became unhappy with her life, having been a lonely sixteen year old for thirty-five years, and befriended her adoptive father's apprentice, Mortimer, to that end. She also took to reading about other girls in the library. After Mortimer, or "Mort", left Death's service, he and Ysabell married and became Duke and Duchess of Sto Helit, as they had previously saved the Queen of Sto Lat.
The couple had a daughter named Susan Sto Helit, but Ysabell and her husband sadly died after the carriage they were in plunged into a ravine, killing them.
It is likely that Ysabell may have lived somewhere in the vicinity of the Great Nef desert. When she was a child, her mother and father were killed in the desert, and Death, who had presumably arrived for her parents, took pity on her and took her to his domain.
He effectively adopted her, and she was raised in his domain with Albert, his manservant, cook and butler. As no time passed there, she remained sixteen for many years.
She was known to enjoy horse-riding, was rude to Albert sometimes, and came to loath the world that she lived in, as nothing was real, just copies of real things. She also got lonely, having only her adoptive father and Albert for company.
Ysabell spent thirty-five years living with Death in his Domain. However, since time does not exist there she remained sixteen years old, as Death believed that children of this age would be easier to handle - showing that even anthropomorphic personifications can get things wrong.
Because of her role in saving the life of the Queen of Sto Lat she was given the title of Duchess of Sto Helit.
Ysabell and her husband were killed in a freak accident when their carriage plunged into a ravine. When Susan grew upset with her grandfather for just standing by and watching, he stated that he had offered them immortality but they had rejected it of their own free will.