Shinka is friendly towards Rikka despite being irritated at times with Rikka's chūnibyō. She also helps out Rikka in situations regarding Rikka's romantic relations with Yuuta. Shinka is the master mind behind all plans for rikka and Yuuta.
Shinka initially hid her true colors from Yuuta, whilst Yuuta initially admired Shinka. She "dated" Yuuta once because she wanted to use him after seeing him getting his hands on the Mabinogion but immediately became distant with him after she found the Mabinogion and having her secret identity "Mori Summer" revealed in front of him, Yuuta also lost admiration towards her at that point. Even then, they still get along because they share the same experience, and Shinka would help him to improve his romantic relationship with Rikka.
Shinka and Sanae have a troublesome, often hostile, relationship. Sanae admires "Mori Summer" a lot but doesn't believe that Shinka is the real one, thus calling her "Fake Mori Summer". Later, Kumin reveals that she thinks that Sanae actually wants Shinka to be the real Mori Summer. Despite Shinka and Sanae's relationship being so troublesome, Shinka still values her and all her friends and wants the best for her.
In the spinoff novel set in the future, Dekomori appears in a cameo as married to Nibutani. However, this may have been Yuuta having a bad dream since Dekomori still refuses to acknowledge that Nibutani was ever Mori Summer. It is left unclear by the end of the novel.
Satone and Shinka had known each other in middle school. Sanae took part in Mori Summer's ceremonies with her. They were partners too, and Satone used to admire her a lot. Satone and Shinka are still good friends, although Shinka lost her chuunibyou self while Satone still keeps hers.
Isshiki was one of those boys who admired Shinka because of her beauty and politeness. He rated her as "Most Beautiful Girl" in a beauty poll of the girls in their class. After finding out her real personality, though, he lost his admiration for her just like Yuuta.
Shinka gets annoyed at Kumin because of her air-headed personality a lot, and often hits her. But they still have a friendly relationship, and Shinka loves her as a friend, along with the others.