I do believe the extra letter has to do with when they are bestowed with horns and whatnot. The placement of the letter is at the beginning for males, and in the middle for females.

As Yru mentioned, we clearly hear "E-Sumi" in one of the cutscenes, so we could probably assume the others follow similarly..that is, the letter is treated as an extra syllable in their name.

Kan-eh-Senna
Raya-oh-Senna
Ah-Ruhn-Senna
Oh-App-Pesi
Eh-Sumi-Yan