In order to unlock Yuna’s Magicite, you must defeat 10 Eidolons in total. They do not have to be different Eidolons; you could just grind by taking on Ifrit 10 times if you like. Once you’ve taken them all down, just go talk to the Migrant Moogle Merchant on the eastern side of the hub (he’s the moogle that isn’t there all the time) and he’ll hand over Yuna’s Magicite.
It is important to note that you must have the Trance ability learned before you can obtain the Magicite and transform. While in a Trance as Yuna, you can use the Great Whirl technique if it appears in the Crystal Surge ability list.
Yuna first appeared in Final Fantasy X as a Summoner-in-training, on a pilgrimage to defeat Sin and bring the Calm to Spira once more. Her journey continued in Final Fantasy X-2; two years later, she is a High Summoner in search of a young man resembling Tidus, her love from the first game. Her design in Final Fantasy Explorers is based off of her Final Fantasy X-2 outfit. Final Fantasy X and X-2 placed 3rd and 9th respectively in our ranking of the best Final Fantasy games.
Check out more tips and tricks at our Final Fantasy Explorers Wiki Guide!