Undines are spirits with the ability to control water. With their powerful abilities and beautiful appearance, Undines are quite popular among 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.
Although Undines are usually technically nude, Summoners can customize their Undines with different costumes and are jokingly 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' 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.
- Undine is the term used to describe water elementals in the works of 16th century alchemist Paracelsus.