Suzu is generally calm, affable and knowledgeable in many subjects.

Spoiler

She appears to be the same age as her classmates, but in truth she suffers from a disease that stopped her from ageing. Thus, she is actually over one hundred years old. This condition gives her different priorities than most people, as she seems to care more about entertainment than the actual success of her assassination attempt. Long ago, Suzu cared for a very special person. He was one year younger than her and their birthdays were only one day apart. They used to laugh about how people don't stay the same age forever but as he grew older, she stayed the same. This caused him to abandon her, which made her resent her condition.

(Source: Akuma no Riddle Wiki, edited)