From the experts:
Marilla is a names that's familiar via its resemblance to Mary and variations, but also distinctive: It hasn't been on the Top 1000 since the 1800s and was given to only nine baby girls last year. Delicate and sweet, which began life as a short form of the flower name Amaryllis. Marilla Cuthbert was a character in L. M. Montgomery's Anne of Green Gables.
Not heard much since the 1880s, Marilla could make a distinctive alternative to Marissa.