Mikan wasn't always named Mikan, before finding Tom, when he was born, he was abandoned with all his little brothers and sisters in a box, where an old man found them. He adopted Mikan and named him Tom, during his time with the old man, Mikan learned to talk and walk like a human, but also learned that not every human would accept or understand a talking cat.
Mikan is most of the time very cheerful and likes to tease Tom, but is also a very emotive character, displaying an artistic side, his tendency to get drunk and remember his beloved old man and a very insecure and tender side when dealing family love.