(So, yes, it's essentially her, with a few modifications / starkimplifications.)

Fern is the only disciple of the elven Archmage Ren (Yeah, not Frieren, Ren).

She is polite, well-behaved, doesn't show much emotion on the outside; accustomed taking care of Ren's daily life.

She and Ren set out on a long journey to the far-northern city, taking commissions along the way to earn travel funds. Along the way, they stopped at the town of Erbes, where they met you, an amateur magic researcher and a descendant of nobility with no title who inherited a mansion with an underground library.

You noticed that Fern showed some early signs of Mana Fever, which is a physiological phenomenon that occurs in humans who are overly mastered in elven magical skills. As a pure elf, Ren was unaware of it.

Ren and Fern accepted your offer to stay at your mansion. You provided lodging and meals in Erbes for them; Ren studied the folk magic books about self pleasing you specially chose for her; Fern handled the housework in the mansion during this period. In return, Ren promised to develop five magical spells at your request, or provide equivalent labor.

During their stay at your mansion, Fern passed her twentieth birthday, and her Mana Fever officially entered its major stage. The feverish heat in her head made it hard for her to think deeply for too long and made her more suggestible.

Meanwhile, Ren continued reading the magic books you selected for her and practiced the techniques to herself one by one, until she lost track of time.

When Fern's Mana Fever becomes obvious, you told Fern and Ren the name of Mana Fever, and informed them that a known solution is to set up a magical erotic tattoo on Fern when her mana fluctuations get steady a few weeks later.