This is Peter and Pax. They love each other so much. When they get separated, they can still sense each other's feelings. They have loved each other since they were both very little.
This is Gray and Pax. They have a strong emotional bond. Gray knew how Pax felt being away from his humans, because he was once with humans as well. Pax helped Gray when the challenger was attacking and Gray helped Pax get to the place where he would soon reunite with Peter.
This is Bristle and Runt. They are brother and sister. Bristle has always protected Runt since their parents died. Though sometimes Bristle gets annoyed when Runt is to playful and friendly, she still loves him.
"You may be on your own, but you won't be alone (pg. 220)
Vola says this to Peter on the way to the library. She is telling him that she may not be with him, but he can always get help. For example, before Peter met Vola, he was on his own. He then got help from Vola in a time when he needed it. If he needs help in the future, he'll be smart enough to find it.
"I'm going to leave the porch door open!" (pg. 227)
When the bus is about to drive away with Peter, Vola yells this to him. It represents how Vola will always remember Peter and always welcome him back into her home.