Meanwhile, back at school, You ends up joining the Astro Circle (Tenkuru), which is in desperate need of new members and has been demoted from an official club to a hobby circle. Its members include Asuho Minahoshi, the daughter of a local cafe owner, Kosame Hisakaki, the younger twin sister of student council member Komomo Hisakaki, and club president Haruto Okaizumi, along with third-year student Suwa Setsuna, who rarely comes to club meetings. Setsuna is implied to be investigating the lookout where Mare appears, for unknown reasons. At night, You continues to visit Mare at the lookout, all the while trying to remember the name of his childhood friend. The Astro Circle eventually recruit You's neighbor Isuzu Aoi by recreating the southern night sky in a homemade planetarium, due to Isuzu loving the southern constellations, resulting in their elevation to official club status.
While visiting the lookout with one evening, You, Chinami, and Mare are attacked by an unseen figure, implied to be Setsuna. Chinami is knocked unconscious but recovers; subsequently, the attacker texts You and asks to meet him at the lookout. The attacker is revealed to have been Kosame, whose intent is to eliminate Mare, whom she describes as an "illusory dream." However, before she is successful, Komomo intervenes, and Kosame later apologizes for her actions, which You forgives.
Hoshizora no Memoria features seven alternate endings depending on the player's choices throughout the game, all of which begin after Kosame's attack. In Asuho's route, Asuho confesses her love for You, but as the two begin dating, she is held back by her fear that You loves the lookout girl from his memories more than her. Asuho, who also knew You in his childhood, has kept with her a Tanabata card written by You on which he wished to reunite with Yume, the lookout girl. Determined to prove that he loves Asuho more, You has Mare destroy the card and thus reap Asuho's nightmare.
In Komomo's route, it is revealed that Kosame is in fact not properly human, but is an illusory dream of similar nature to Mare. Kosame died in an accident at the lookout during her childhood (explaining why the lookout is fenced off), prompting a grief-stricken Komomo to use her powers as a shrine maiden to give her new form as an illusory dream, using the shrine's sacred object, a meteorite, as her host object. Kosame is revealed to have attacked Mare in order to protect Suwa, and furthermore is the one who requested that Suwa be sent to Hibarigasaki, in order to eliminate Kosame herself. However, Suwa, who has grown to care for Kosame, ultimately refuses to carry out her task. Kosame then asks Mare to send her back using her scythe, but You and Komomo intervene. Komomo instead asks Mare to reap her own nightmare, which turns out to be Kosame's ephemeral nature. As a result, she is able to remain in human form.
In Kosame's route, it is revealed that she is unable to retain a physical form while under the light of the full moon. When You discovers this, Kosame expresses sadness about her unreal nature and explains that they can never be together, but You, unable to accept this, is able to cause Kosame's physical form to return. However, Kosame, still unable to accept that she is anything but a ghost destined to disappear at any time, ultimately asks Mare to eliminate her. Mare, however, criticizes her for this and points out that You would be grief-stricken if she were to depart. To prove this, Mare instructs Kosame to vanish, then tells You that she has "returned." You, seeing through Mare's deception, borrows her scythe and cuts his wrists, causing him to enter an oneiroid state that allows him to perceive of Kosame, who tearfully recognizes You's love for her and decides to stay. You is later taken to a hospital, during which time he has a vision of his mother, Kasumi, who says she no longer needs to watch over him.
In Isuzu's route, You learns that she was in fact raised in Australia, but moved to Hibarigasaki with her parents due to her younger sister Suzuha's fragile health. Isuzu, devastated after moving away from her childhood home, began to visit the Hibarigasaki science center to see planetarium shows of the southern night sky. After the science center closed down, its director gifted Isuzu with an old telescope, along with a key to Hibari High's rooftop, so she could continue stargazing. While stargazing one night with Chinami and Suzuha, Isuzu and You are visited by Mare, who recognizes Isuzu's "nightmare" and reaps it with her scythe, causing the telescope to fall apart in Chinami's arms. Isuzu blames Chinami for breaking the telescope and stops talking to her, which results in You seemingly distancing himself from Isuzu as well. However, Chinami and You later give the repaired telescope back to Isuzu, revealing that they worked together to fix it in order to mend their friendship. Isuzu forgives Chinami, and reconciles with You as well. Years later, You takes Isuzu to Ayers Rock in Australia to see the southern night sky once again.
