Starting off with Yotsuba. She gives off a really fun and energetic vibe that you just can't deny. The presence of Yotsuba is happiness itself. She willingly goes out of her way to support her sisters the best wat she can and only wants her sisters to be happy.
Yotsuba is also the first quint who supported Fuu as he struggled from his tutoring and did her best to ensure that he is able to help her sisters. She does not let her feelings decide her actions because she wants everyone to have a genuine and unbiased chance at romance.
Following this, what makes her the quint I love the most is how selfless she is as she continues to put her sisters first before herself. Pushing Fuu forward to encourage him to achieve his goals, and helping her sisters come to terms with their troubles, to name a few.