The Six seem to be all friends, as they live in the same house together. However, most of them argue or don't get along well frequently, especially and specifically with Nickel (such as Nickel's continuous arguing with Puffbull or his conflict with Cup on the bus). The characters are placed on different teams:
Team A: Cup, Nickel, Puffball
Team B: Penny, Puffbull, Tennis Ball
It is possible that half of them are Team A and the other is on Team B for a reason, or just to even it out.