Is Identity client side? Just so I know if I can troll my friends with it.

No, Identity is required on the server and client. Anything that your friends see would have to be done on the server, because the client cannot tell other clients what to do or see. You may have better luck looking for a server-side spigot/bukkit plugin that adds morphs (because then they do not have to install anything). Good luck in your trolling adventures!