Finally we have a release date for when we can expect to see Medli arrive in both versions of Hyrule Warriors and that date is May 19th for Japan.
No date has yet been given for the western release of Medli and the Wind Waker Pack dlc which also releases on the same day, but the current assumption is it will be either on the same day, or within a couple days after. News of this comes thanks to Koei Tecmo, who released a trailer for the first dlc pack for Hyrule Warriors Legends. You can find the trailer below which showcases Medli, the new Adventure Mode Map (a Master Quest version of the Great Sea Map,) as well as new costumes for your warriors and your Fairy companions.
As soon as we know more regarding a western release date, we will be sure to fill you in as always.
Source: KOEITECMOofficial (YouTube)