Chitaru is a kind and caring girl who really looks out for others, especially those that appear to be heavily dependent (such as Hitsugi and Haru). She states early on that she has no plans to target Haru whatsoever. She appears to be quite attached to Hitsugi, but mainly in a motherly way.
It is revealed in chapter 18 that she joined the Black Class in order to avenge her former teacher, who was killed by an assassin named "Angel Trumpet." It is heavily implied that Hitsugi is somehow affiliated with Angel Trumpet.
In Episode 6 Chitaru after hearing that Shiena Kenmochi is poisoned, concludes Azuma Tokaku is the Angel Trumpet and attacks her during the school play. Hitsugi stops her from fighting Tokaku any further by revealing she is the Angel Trumpet. Conflicted at carrying out her duty to her teacher to kill the Angel Trumpet and her love for Hitsugi, Hitsugi forces Chitaru to kill her.
After being told by Nio that Hitsugi wanted to live with Chitaru after assassinating Haru, Chitaru swallows the bottle of poison and dies with Hitsugi at the conclusion of Romeo & Juliet, revealing her ultimately romantic feelings towards her.
In the final episode, it's shown that she and Hitsugi are still alive, even though they appeared to die in episode 6.