Charles is her father and he is the only one who knows the fact she has two bodies. He also ordered her to keep an eye on Daniel, presumably because he suspects Daniel has two bodies as well.
Kouji and Crystal know each other well because Kouji is one of the ten people who were raised by Crystal's father, the HNH Chairman. These ten special children are considered prodigies under his care. Even though the two know each other substantially well, they hardly meet unless Crystal needs Kouji's assistance in an emergency or any similar matter. Their relationship is based on money as he will only do her bidding if he makes a profit off of the work.
DG and Crystal are close acquaintances and know each other quite well because of Crystal's father, the HNH Chairman. His friendship with Crystal surprises many girls since he doesn't show any obvious kindliness to any other girl. Despite being very close to each other, DG and Crystal have no romance in their relationship.