Joshua is a student representative. He is an extreme gentleman and very kind. He has a soft heart and it seems he has taken a great liking to Yuuta. Joshua tends to always be there to break everyone up from fights.
Joshua seems to be a distant relative of King Alphonso, as the painting of Alphonso's coronation hanging in the school hall belongs to Joshua's family. Besides, he and Alphonso look alike. However when everyone else thought Yuuta stole the diamond, Joshua did not suspect him at all.
Judging from the way things happened in episode 1, Joshua's mother died. The reason the diamond had been so important to him was because it was the color of his mother's and his own eyes.