The answer to these questions can be found on p. 13 of Call It Courage. Mafatu’s closest companions were two animals, a dog and an albatross. He made them his friends because they were different, just as he was different.
By the time that you get to p. 13 in the book, you will know that Mafatu is something of a reject on Hikueru. He is the only boy who is afraid of the sea. Because of this, he has a hard time doing any of the things that men are expected to do on that island. The boys scorn him because of this and generally stay away from him. He does not have friends among them.
Since humans will not befriend him, he turns to animals. His dog, Uri, is his “inseparable companion.” The “only other true friend” that Mafatu and Uri have is “Kivi, an albatross.” It is clear why Mafatu chooses Kivi. Kivi’s feet are different sizes and one leg is not fully formed, making him different from all the other birds. They pick on him. His efforts to fight back “touched the boy’s heart” and Mafatu raised Kivi to adulthood. It is less clear why Mafatu picked Uri in particular or even if he did. However, Uri did have a “thin coat which showed his ribs,” which implies that he is not a strong and handsome dog. Mafatu identifies with these animals because of their imperfections. He cannot find human friends, so he sticks with his animal friends who, like him, have problems that set them apart from their own kind.