As in the case with Kento, I think Sage has been given an injustice. A bad reputation as a womaniser. I used to be ok with reading a fanfic that made him a pig, but then after a while, it bothered me so much. I can't even read one that hints at him being such a jerk.
Just by looking at his clothes, you can tell he grew up in a house hold where manors are important. He dresses neat (I don't mean as in 'that is so neat'). The boy wears a plain white shirt with itchy looking pants for goodness sakes. This shows that he has been raised with manors and in a house that was more strict than most. I think he wears the clothes he does because he was raised that way. Not that it looks BAD, just uncomfortable. His home life must have been filled with rules. This is also backed up by the fact that his father owns a dojo, and Sage learned things from his father at the dojo. From what I hear, those teachers are strict, and rules & order are important. You must be obedient.