Chiaki is Shizuki Ryouko's childhood friend, having known her since primary school.
Prior to Ryouko's departure from Earth, Chiaki appears to be her closest friend, belying Ryouko’s rule against making friends with normal, earthbound interests. A violinist, she is well used to Ryouko’s quirks.
Notably, she goes out of her way to encourage and aid one of Ryouko's male suitors, despite probably knowing about Simona del Mago's crush on Ryouko. It is unclear if this means anything, though. Her stated reason is that she is trying to give Ryouko an attachment to come back to, to increase her chances of her survival. This indicates that she may have lost faith in Simona's ability to form such an attachment.
Chiaki is a serious, practical girl, with little understanding of Ruiko’s whimsiness, Ryouko’s staring at the sky, or Simona’s inability to spit it out with regards to her crush. Though socially awkward in her own way, she is not as reclusive as Ryouko is, and views her with some degree of protective affection. In many ways, she treats Ryouko like a younger sister, consciously serving as the implied muscle if she ever thinks Ryouko is being bullied, and can be easily imagined taking serious suitors to the side to lay ground rules, with the usual implied threats in case of mistreatment. Of course, she is also fond of teasing Ryouko to distraction, unintentionally fueling Ryouko's body insecurities, since Ryouko envies her height and seemingly greater physical maturity.
Chiaki is the tallest of any of Ryouko’s friends, with long hip-length hair and a slim, almost reedy build, though she imagines herself to be stronger than she appears.