The Nymph Banner is a banner dropped after every 50th kill of a Nymph. It is displayed in the background, similar to paintings, only hung from the ceiling instead. Each banner gives a small bonus to defense from and damage towards the creature it is designed after.
It counts as both the Lost Girl banner and the Nymph banner. (it shows the Lost Girl and the Nymph on different sides)
Due to the rarity of Nymphs, the Nymph Banner is arguably the rarest banner in the game.