Yumemi is a goodhearted but extremely talkative gynoid attendant of an abandoned planetarium; she is designed to look like a young girl. Yumemi is slightly damaged and completely unaware of the changes that have occurred in the past 30 years, as none of the facilities and databases that she connects to exist anymore. Therefore, she treats the protagonist like a regular guest by calling him "Mr. Customer" (お客様 Okyaku-sama), speaks of the world as it was before the war, and fails to understand any information he tells her, other than things related to her job at the planetarium. The name "Hoshino Yumemi" itself is a pun—"hoshi" means star or planetary body; "no" is a possessive particle; "yume" is a dream or a reverie; "mi" means see. Yumemi is the only character shown to the audience of the game.
Yumemi is very adamant about protecting humans and is happiest when she is helping those she serves. When she is unable to help someone, she gets terribly worried that she is incapable of offering assistance and must instead indirectly help a customer by directing him or her to someone who can. Protecting humans is her top priority and will even ignore previous orders to make sure no human is harmed when in her care.