[17:57] <@Ryllharu> No way. The devil in that partnership is the teacher.
[17:57] <@Ryllharu> The loli is something else entirely,
[17:57] <@Ryllharu> Maybe a Weiss that betrayed the rest of them?
Chapter 113 page 8

Akari: His name was... umm... was it Katsuragi?
Nikaido: Oh... its rare for Rimyuel-dono to remember the name of a human.
(etc two more lines)
Akari: You always talk so haughtily... You are able to use your power because of me!!
Nikaido: Pardon me, pardon me.


So yeah... Nikaido also is the school friend of Jun. Nikaido is definitely human, but is able to use the power of devils thanks to Akari, named Rimyuel, who probably is indeed Elsie's sister.