lucina_(summer)_(fire_emblem)
Tag used for Lucina (Fire Emblem) from Fire Emblem specifically when dressed in her swimsuit from the Fire Emblem Heroes 2020 Summer Passing banner.
In-game, she shows up with her mask, and presents herself as Masked_Marth, so the corresponding tag should be added.
Extremely likely to see her along with Summer_Mia, the main character of the Duo Unit she's part of - Summer Lucina isn't her own unit in-game.
Note that it's possible (especially for older artwork), to depict Lucina in a swimsuit without it being the one described above, in which case this tag should not be used.