Koji is Penny's older brother, with the siblings exhibiting mutual animosity for unspecified reasons. He belongs to a trio alongside Marvin and Ted, collectively identified as Penny's companions. Despite their typically antagonistic relationship, an event occurs where the experiment Babyfier transforms Koji into a toddler; during this time, Penny displays authentic concern and sadness for him, contrasting their usual dynamic. Later, Koji develops a romantic affection for Sae, a transfer student joining Yuna's class in the second season. This affection remains unacknowledged by Sae herself. Beyond these interactions, Koji's background, development, and narrative contributions are not explored further in the documented media.