For the basic plot of this anime, it'd be best if this anime page had the genre of Slice of Life instead of Romance.
Sayu is a spoiled brat who made wrong decisions before meeting Yoshida, and Yoshida, trying to be a proper adult, tells her off and tries to get her back to becoming a normal teenager.
This can not be compared to KoiKimo, another anime that aired at the same time as this, and is about a huge age gap. Here, the romance genre is about Yoshida and his romances with his colleagues and has nothing to do with Sayu. Sayu, being spoiled, tries to make advances on him for letting her stay with him, but she's soon told off on every such occasion.
I wouldn't call his relation with her as that of a brother or a guardian, but just as a normal & responsible adult who knows what's right and wrong for a teenager who has lost her way in life.
