Shiena seems to have a normal personality compared to the rest of the members of Class Black. Although she is somewhat scatterbrained and forgetful, she is also strict at times, as shown when directing the play in Episode 5. She says she will do anything to protect the place she belongs to. Shiena is skilled at analysis, as seen when she is able to gather information about Hitsugi and Chitaru by simply watching them during rehearsals for the class play. She is also a skilled hacker as seen in the final episode where she successfully hacks into Myojo Academy's system.

Spoiler

When Shiena was about to submit her assassination warning letter, she was poisoned by Hitsugi and fell unconscious. Haru finds the letter beside her unconscious body, but it was invalid due to her condition and later hospitalization. Thus, her assassination attempt was counted as a fail, and she had no choice but to be removed from the competition. She is the only assassin to fail an attempted assassination without even starting it. When she was younger she got bullied a lot. She had one friend whom she talked to on the phone called Yukarin, but one day she asked Shiena if she wanted revenge on the people who bullied her. Yukarin told Shiena about the group called Collective Dismissal and that they will give her her 'vengeance'. Shiena asks what they want for in exchange and Yukarin tells her that she must avenge someone else and Shiena accepts.

(Source: Akuma no Riddle Wiki, edited)