Here are some possibilities:
* If we're talking about the very first animated characters Disney created, it was likely Oswald the Lucky Rabbit. He predates Mickey Mouse, appearing in 1927, but the rights were lost to Universal Studios.
* If we're talking about the first Disney character to gain lasting fame, that would be Mickey Mouse, who debuted in 1928.
* If we're talking about the second character to appear alongside Mickey Mouse in a cartoon, it would be Minnie Mouse, also debuting in 1928.
So, there's no single, definitive answer to your question. It depends on how you interpret the terms!