The Friendship Trail is a starry trail that is said to come out of a child's heart when they are in need of a friend, specifically, a pet, to help them.
The trail would usually enter into the Magic Fountain's waters to make the picture of the child appear as well as crystallizing the water. One of the Royal Guards would then notify Princess Ava about said event and Ava would then consult the Pocketpedia over what animal will become the child's friend.
While Ava was away in the Big City and Kate took over her duties, the trail enters into the Friendship Heart to let the user know (For Kate's half, the child, and for Eva's half, the Chosen Pet, respectfully). It seems that only Kate and the citizens of Pocketville can see it.