Jelsa is popular on the web, but you are aware that the Jack Frost version all this shipping is based on is not a Disney character but an animated character by DreamWorks Animation/Paramount Pictures based on a book series by William Joyce? And no, these books are far from being public domain, they just have been published. Of course though there could be a Jack Frost, he is a folklore character as the spirit, personification of winter, but that would have to be some very different character to avoid any copyrights issues. So, not the Jelsa you might be hoping for.
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