Race Human Leo is one of the kids who were granted immunity by the Pope because he was the one who designed and who led the project in order to build the ark. He uses crutches to walk or is carried by someone else because he has bad legs which he had since he was a baby.

Spoiler

It is revealed in chapter 10 that Leo is actually Locke, Chloe's long lost brother, although he himself does not know as he was still a baby when he fell down from the wall.