Mitsuki Ichijō acts as secretary to Kozo Ryuzoji, headmaster of Sakaimachi Academy, having previously taught history there. Her surname Ichijō means "one article," and her given name Mitsuki blends the characters for "beautiful" and "moon."

She shares a complex history with Takuya Arima, stemming from a past romantic relationship born partly from her emotional void after Kodai Arima's presumed death and her own loneliness. Though their romance ended, mutual affection and respect endured. Mitsuki harbors deep, unrequited feelings for Ryuzoji, her former college professor who is married. This yearning deepens her emotional vulnerability and sense of isolation. Her connection to Takuya provokes jealousy in Mio Shimazu.

Observing Ryuzoji's abrupt behavioral shifts, particularly fearful reactions towards an unspecified "she," prompts Mitsuki to investigate the cause independently. Her vulnerability and need for connection make her susceptible to Ryuzoji's manipulation. Using hypnotic techniques, he compels her to retrieve the Reflector Device from Takuya. Under this influence, she confronts Takuya at gunpoint near Mount Sankaku. A struggle ensues between Ryuzoji's control and her lingering affection for Takuya, culminating in her turning the gun on herself. This event occurs within the "Schwarzschild radius of the event," marking it as a fixed, inescapable outcome across all story routes, establishing a narrative rule that some fates remain unalterable despite time manipulation.

Her character design evolved between original and remake versions: originally depicted in a black shirt with purple suit, the remake features a light purple outfit of coarser fabric.