Why does riku dress up as an organization XIII

riku is wearing the organization 13 cloak because long ago when riku had the apperance of a child -because his heart was so strong ansem the wise told him to search for a young man named roxas and bring him to him. and roxas had beaten riku in a fight. so riku decided the only way to win the darkness is to imurge yourself in that same darkness.so when riku brought roxas to him he was inerducing himself as ansem, he also says that he wanted revenge at roxas and riku ended up being the victom of HIS revenge. i hope this makes it more clear! if not try some kingdom hearts websites or just plain wikipedia-type in kingdom hearts charecters! hope this helps :)