A joke from the "Yggdra Unison" drama CD.
In the "Milanor Thieves Chapter" where Milanor is the main character, there is an incident where the "Diamond Dust" card goes off and turns all four party members Milanor, Kylier, Roswell, and Rosary into Undines.
And in the very same "Milanor Thieves Chapter", Yggdra is faced with the choice of whether or not to put on the Mermaid Fin and become an Undine.
When has system quirks being used in-story last been this delicious?
And so, this tag refers to the above five characters, or others, in Undine form.
In the case of male characters, this is also a Female Genderbend, so take warning if you dislike those.