Undine

Undine is a monster in the MMORPG Elder Tale.

Overview
Undines are spirits with the ability to control water. With their powerful abilities and beautiful appearance, Undines are quite popular amongst Summoners who can make a contract with them after fulfilling specific quests. After the Catastrophe, their ability to produce water makes adventurers hold them in high regard.

In the anime, summoned Undines wear different costumes and are comically called cosplayers. It doesn't seem to be a terribly uncommon habit for Summoners, as the customization of their summons is seen to be one of the class's perks.

Round Table Alliance arc
When Log Horizon moves into its new Guild Base, they receive visit from Isaac, who arrives with several summoners from his guild. They summon Undines in cosplay attire who wash out the guild hall (and Naotsugu) with a strong, combined blast of water.

Trivia

 * Undine is the term used to describe water elementals in the works of 16th century alchemist Paracelsus.