Aoi Housen is a sixteen-year-old girl from Earth who serves as a flight attendant trainee aboard the Liebe Delta, a role she continues to fulfill after the events strand the crew and students on the advanced ship Ryvius. She is a childhood friend of the Aiba brothers, Kouji and Yuuki, and has known them longer than anyone outside their immediate family. This long history places her in a unique position, often acting as a caring older sister or mother figure who constantly checks on the two boys, despite the fact that neither seems to appreciate her concern.

Personality-wise, Aoi is defined by her caring and responsible nature. She frequently shows a trustworthy and supportive side, encouraging anxious friends like Kozue Izumi and Reiko Ichikawa during the chaos of their situation. However, she also has a tendency to neglect her own emotional needs and is remarkably obtuse, especially regarding romantic matters. She struggles with direct communication, sometimes speaking her mind without a filter and at other times hiding her true feelings behind a smile rather than expressing them clearly. This emotional reserve prevents her from acting on the deep feelings she harbors for both Aiba brothers, feelings that are apparent to others but largely unrecognized by her until much later.

In the story, Aoi is one of the key figures among the main cast, providing a stabilizing presence for her friends as social order on the Ryvius begins to break down. Her closest relationships are with her fellow flight attendants, Kozue Izumi and Reiko Ichikawa, and the three initially room together on the ship. When Kozue is physically and mentally injured following an assault by classmates, Aoi tries to be a pillar of support, but her efforts are rejected. Soon after, Reiko also leaves their shared room, driven by guilt and fear of another student, Ikumi Oze. Abandoned by her two closest friends, Aoi is left alone and forced to struggle with an unfamiliar and crushing sense of solitude. This isolation leaves her vulnerable when she becomes the target of an assassin sent by another character. Facing a life-threatening situation, she reaches a mental crisis point but finally comes to a realization about her own feelings: she loves Kouji Aiba. Desperate and relying on him, she confesses this truth, which helps her overcome her breakdown. Following this, she and Kouji begin a romantic relationship, eventually returning to the Ryvius as an official couple by the end of the series.

Her development throughout the series is a journey from a self-sacrificing caretaker who suppresses her own emotions to someone who must confront her personal fears and desires. The loss of her support network of Kozue and Reiko forces her to face her own emotional fragility, and the assassination attempt pushes her to finally admit the truth of her feelings for Kouji, leading to a more honest and direct version of herself. She does not possess any notable combat or piloting abilities, as her role is primarily that of a flight attendant, focusing on the care and service of others.