I don't read japanese so i dont know exactly what the secret ending says but i did see one part where it quotes some of the ansem reports. I don't know what part is quoted but from what i remember about them there was one that mentioned ansem's encounter with someone from another world and that that someone was a king in that world (no, i do not think it is mickey). Even before i saw the secret ending and before kh2 came out, while reading the reports it just didnt sound or feel like it was mickey. Anyways, i think the king that ansem talks about in his reports is the tall knight and the reason he took mickey's keyblade is because he is a king just as mickey was.