Kōki Saiki is a 20-year-old university student working part-time as a waiter at Wagnaria Restaurant. He stands 175 cm tall, with short chestnut-brown hair and dark brown eyes. While the original webcomic states he was raised in South Korea, the anime adaptation omits this background detail and instead depicts him having limited proficiency in English rather than Korean. This significant language barrier profoundly impacts his early work experiences.

His initial inability to communicate fluently in Japanese causes considerable nervousness and social anxiety. He struggles to interact with most colleagues, leading to isolation. Senior waitress Kisaki Kondou becomes his essential communication link, as she is the only person he can reliably understand and converse with. This dependence results in him blindly following Kisaki's instructions, unaware she exploits his naïveté to delegate her tasks, effectively turning him into her personal errand runner.

Over time, Saiki develops greater confidence in his language abilities through workplace interactions and gradual immersion in Japanese. This linguistic improvement coincides with emotional growth, transforming him from an apprehensive newcomer into a more self-assured team member. While still characterized by inherent naïveté, his evolving communication skills reduce his reliance on Kisaki and facilitate better integration with other staff.

Within the restaurant's dynamic, Saiki is portrayed as one of the more grounded employees, providing a contrast to the exaggerated quirks of his coworkers. His initial communication limitations position him as a relatively sane presence, though his evolving role allows him to participate more actively in workplace activities as the series progresses.