Reiko
While Yohko wasn't all that concerned about the demon realm in
the  first  place,  she  certainly  wasn't  expecting  that  one  of  her
classmates  was  plotting  against  her!  Reiko  is noted for being
graceful,  often   carrying  a  rose,  and  always  looking  "perfect".
Once Reiko aspires to become the black queen however; she is
pitted directly against Yohko.  

Osamu Abunai
Osamu is a guy that Yohko has seemed to have known for many
years.  Osamu however has sort of grown into a crush on Yohko.
Unfortunately  while Yohko has a crush on just about every other
guy, she sort of ignores Osamu who is sort of.. socially impaired.
Although it would  seem that he would  end up a main character,
Osamu only appears in the first episode.

Principal Gakuencho
The  principal  of  the  school  that  Yohko  goes to  isn't  really  known
for  being  easy  going.  In fact she  carries  a whip  around (or usually
does when she starts going on an "S & M trip".  The main plot against
Yohko seems to be the  doing  of the  principal  who is a demon (or is
possessed by one - not sure on that).

Hideki Kondo
Hideki  is a very  minor  character and plays  more of a role in Yohko's
fantasies  than in the  actual  stories  (in fact  I think the only place that
he  talks is in  Yohko's dreams).  Rather  unfortunate for  Hideki, is the
fact  that he's  been  chosen  to be  sacrificed to  Reiko.  Surely  Yohko
won't let a hunky guy like Hideki get sacrificed to some OTHER girl!