Saiko Yonebayashi is a Rank 2 Ghoul Investigator, a Quinx, and a member of the Quinx Squad.
Saiko has two thick sections of hair worn in pigtails with uneven fringed bangs, possibly as a result of her laziness. Her kakugan is located in her left eye because of her status as a Quinx, and her face appears rather youthful; Saiko’s true weight is unknown but is believed to be dangerous for her occupation, and she is rather chubby, with notably thick and chubby legs, she also appears to have big breasts. She is very short, especially for her age.
Saiko dislikes having to work, as she prefers having a peaceful, happy-go-lucky and carefree life by playing video games and eating junk food excessively. She often oversleeps, which earns her the absence from most missions. While not trying to reduce it, she is apparently embarrassed about her weight, humorously covering it during her introduction.
She is excessively lazy to the point of being a hikkomori, unable to remove herself from her room without the intervention and often force of others, even jeopardizing her occupation. Saiko is also depicted as greedy, and heavily involved in her videogames to the point of escapism.
She appears to have very acute hearing ability.
It is later revealed that she was forced into joining the Quinx Squad by her mother in order to earn money. When Sasaki is questioned by Ginshi over why he hasn’t fired Saiko yet, he mentions that she scored the highest mark in the aptitude test for the Quinx Squad.