In the Japanese version the first deliberate use of the name "Mickey" comes at the end of Re: Chain of Memories Reverse/Rebirth. Upon defeating Ansem in his heart, Riku finds Mickey on his way out and calls him 王様 to which Mickey indicates a certain embarrassment. As his friend, Riku then calls him Mickey.
(Riku and Mickey at the end of Re: Chain of Memories)
This link between Riku and Mickey is followed up throughout Kingdom Hearts II and especially at the end where Mickey hugs Riku while Donald and Goofy hug Sora.
(Reunions at the end of Kingdom Hearts II)
Why is it that Mickey as the main character of a certain franchise is not depicted fawning over the nominal lead character of Kingdom Hearts, but rather, the dark hero Riku? Is this deliberate beyond as a simple plot choice to allow Riku 'friends?'