Sextum

Sextum (セクストゥム Sekusutumu) is the Averruncus of Water and is the only female of the Averruncus series. She is shown to have more respect for Dynamis than Tertium does.

Plots
Sextum first appeared in chapter 311 in the manga, after Quartum cutting Chachamaru Karakuri's body in half and Quintum taking the Great Grand Master Key from Nodoka Miyazaki, she trapping Chisame Hasegawa and her friends into the water to killing them.

ANIME FINAL
Sextum make a cameo appearance in beginning of the movie.

UQ Holder!
Sextum make a cameo appearance in Chapter 139 of UQ Holder!.

Trivia

 * Like her companions, her name is derived from numbers, and in this case from the latin word for "sixth", as she is the sixth generation Averruncus.
 * Sextum seems to hate turtles/tortoise.