Helen was my surrogate singer, as I have said often! I made her a soprano, for many reasons. In my imagination, sopranos have one sort of personality, and contraltos have quite a different sort. Sopranos are dominant, brilliant, tall, beautiful, have a powerful need to be loved; have a strong sex drive. They're charismatic, highly strung, and have a powerful drive to succeed.
Contraltos (altos; contralto is the word used for soloists) seem to me to be more retiring, more earnest, more serious, more nurturing, more concerned with success of the group, and less with their personal advancement.
In the Helen story, I did not stick to these—personal—stereotypes. Yes, Helen was brilliant, tall, beautiful, a little needy, and somewhat highly sexed; but I gave her a strong sense of responsibility, and self-control. Helen was given less of a drive for personal success, than for achievement, more broadly.
In my incomplete story about the Weihnachtsoratorium (the Bach Christmas Oratorio), there are two soloists: the contralto Maria Schreiber, and the soprano Amalie de Groot. These two characters embody my personal prejudices much more closely. Amalie starts off being somewhat immature, but grows throughout the story. The contralto is delightful, and I have put a little excerpt about her in one of my Blog pages.