Yui demonstrated in the recent chapters that she is very intellgent and strong, because of this she managed to reunited the Char Siu clan. She has a really interesting story that I'd like to hear more. Although she is like this and cheerful everytime, she is lonely as well, because she lost her parents recently and the only one who she considers a parent alive now is Raku. She really conquered me in a short amount of time.
Besides Chitoge, who has the most natural relationship with Raku, she is who has the most natural, if Raku didn't see her a sister, she would have a chance in the harem. Marika relationship with him is unnatural and Onodera is forced.