nop leather is not necessary, you can use a fake leather (tissue with leather look), it'll goes very well for clothes, and it's more simple to work with it.
if it's for an armor or for armor part, that's different, the real leather is better to give the real armor look, but fake leather can also be used for armor, just need to make it heavier in the apperance and in thickness, by using multiple layers or using another materials to put under and give to your project the heavier apperance it needs.
hope it'll help you, good luck for your first cosplay