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As a child, Ysabell had been rescued from death by, ironically, Death himself (probably in a rare moment of sentimentality). When she was 16, Death brought her into his own realm outside of time and space, where she lived as his adopted daughter.

During all that time, Ysabell didn't grow old, as nothing ages in Death's realm. But living in this stale realm, with only Death and Albert as companions, was boring her a lot. The situation changed when Mort was appointed Death's apprentice.

Initially Mort and Ysabell didn't like each other, but after Ysabell helped Mort out of the mess he had created with Princess Keli, they fell in love with each other and finally got married. Death released them both from his realm and they became the Duke and Duchess of Sto Helit. Susan is their only child.

After they left Death's realm, their immortality was taken away as well, and they both died in an accident some 16 years later. During their last moments Death offered to bring them to his Domain for them to continue their life, but was turned down because they both decided it wouldn't be truly living, but just postponing their death.