Chisato Mera is a student at PK Academy and a classmate of Kusuo Saiki. She is a girl of average height with auburn hair and red eyes. Her birthday is on April 15th. Mera comes from a family that has fallen on extremely hard times. While her family was once quite well-off, her father's business went bankrupt, leading to financial ruin and significant debt. This poverty is the central driving force behind nearly all of her actions and defines her personality.

As a result of her family's financial struggles, Mera is constantly hungry and is almost entirely obsessed with food and money. She works an exhausting number of part-time jobs, reportedly as many as twelve at one point, to support herself, her father, and her younger siblings. Her most frequently seen place of employment is as a waitress at Cafe Mami, a spot that Saiki Kusuo often visits for its coffee jelly. Despite her hard work, her constant state of near-starvation often overrides her professionalism and social graces. In a notable incident, she served a customer their coffee jelly only to immediately consume it herself, an act that demonstrates her inability to resist food.

Mera's personality is a complex mix of opposing traits. She is a determined and diligent individual who works tirelessly to provide for her family, showing a fierce loyalty and a sense of responsibility. She has even been shown to have remarkable survival skills and a high metabolism, allowing her to find edible plants when stranded. However, her impoverished circumstances also make her appear selfish, tactless, and greedy, particularly when food is involved. Her extreme gluttony can lead her to take food directly from her friends, an act portrayed as a regression to a primitive state during moments of intense hunger. While she is often apologetic after such incidents and can be mild-mannered, her single-minded focus on food frequently strains her relationships.

Within the story, Mera serves as a recurring supporting character within Saiki's social circle. Her primary motivations are to earn enough money to survive and to reunite with her father, who was working abroad in Puerto Rico. A key relationship is with Saiki Kusuo himself. While he often finds her antics troublesome, he also feels a degree of pity for her situation. He has used his psychic powers to assist her on multiple occasions, such as giving her a plane ticket for a school trip and manipulating a teacher's mind to change school rules so she could keep her part-time job. She is also close friends with her classmate Aiura Mikoto, who used her fortune-telling abilities to help Mera locate her father, and Aiura affectionately nicknames her Chisapoyo.

Mera undergoes a degree of character development over the course of the series. Her portrayal is initially more balanced, but later seasons focus more heavily on her gluttony as a primary gag. Her main story arc sees her father return to Japan, and with Saiki's help, the family is freed from the debt collectors pursuing them. Following this resolution, her compulsive behavior around food does not immediately cease, though she does show signs of gradual improvement, such as reducing the number of jobs she works. In a notable later development, she briefly becomes wealthy after an investment in Bitcoin, which temporarily changes her demeanor before her circumstances return to normal. Among her notable abilities, beyond her resilience and survival instincts, is an astonishingly fast metabolism that allows her to consume vast quantities of food without seeming to gain weight.