How to obtain S. Rank characters

So I have been playing Pocket Knights 2 for a few months now and I cannot find any information anywhere about how you upgrade your heros to the S. rank (I think they are called Mythical Heros at S. rank). My question is, how do you go about ranking up one of your characters to the S. rank? Please let me know, I have searched all over the internet and I came up empty handed with no answers and its driving me crazy! Thanks.