In the book The Rise of Nine by Pittacus Lore, Number Nine is travelling with John, the main character.  He is same age as John and from the same planet (both are aliens) but he has a very different personality than John.

John is cautious and responsible; Number Nine is cocky, arrogant and irresponsible.  When the two were on the subway in Chicago, Nine tried to get the number of two girls on the train. This was extremely risky because the girls could be working with the Mogadorians, the aliens trying to hunt the Numbers.  Also, the fact that they went to Chicago was very risky because the Mogadorians can easily blend in big cities and are hard to notice; the Numbers are more exposed in the city to being spotted, too.  John and Nine had to stay together but John really didn't want to go to Chicago because he knew it was dangerous.

John and Nine were very different characters and handled situations very differently throughout the book.  All the Numbers needed to stay together and if it weren't for John, Nine would have been killed by the Mogadorians.