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Yui and Raku share a sibling-like relationship considering the two have been acquainted to one another since their respective childhood. This is evident as Raku affectionately addresses Yui as "Nee-chan", at least when they are alone or at home. Yui greatly treasures Raku for livening up her childhood, considering him as a part of her family and is greatly dismayed when she was separated from him. After losing both of her parents and gaining the title as head of the Char Siu, Yui greatly longed for someone she considers as part of her family and was overjoyed once she finally got the opportunity to reunite with Raku, the last person she considers her family. While it is apparent that Yui cared for Raku like a younger brother, Yui clearly states that her love for Raku extends beyond that of a sibling relationship. As such, Yui tends to spoil and tease Raku, but not to an extent that mirrors that of Marika's 'bold' advances. Furthermore, Yui does not seem to mind being Raku's bride after Ie forces them to be in a relationship; in fact, Yui suggests it as an option for Raku if his career does not succeed. However, while Raku initially felt oblivious of Yui's feeling for him, in the end Yui feels compelled to have Raku all to herself and is willing to fight for it even against Chitoge and the others.
Eventually, after feeling pressured by Ie and Marika, Yui confessed her love to Raku but was shot down as Raku had someone else he liked. Nevertheless, Yui continues to care for Raku, maintains their sibling-like relationship and support Raku's romance.
As her personal aide and mentor, Yui genuinely respects Ie and puts her on high regard, serving as her guardian.
Yui more or less likes Chitoge for her overly cheerful and likable personality. Chitoge was one of Yui's childhood friends and as such, treasures her. While she has no ill-will in regards to her being Raku's current girlfriend, Yui declares that she has no intentions of losing over to her in winning Raku's love. Aside from these, Yui and Chitoge are good friends while being love rivals.
While Marika somewhat resents Yui for 'interfering' with her advances with Raku since their childhood, Yui continues to do so in the present in every chance she has. Nevertheless, Yui genuinely cares for Marika.