Fairy Ring Mushroom is the first Legendary plant obtained by the player after beating Floor 21 of Devour Tower along with Night Cap. Her seed packets are more likely to be dropped in Shallot Graveyard breakout levels.
When any zombies are defeated by her attack, the zombies are transformed into a Puff-Shroom in a similar manner to Witch Hazel. This attack manner is also similar to Spore-shroom when it defeats a zombie.
Fairy Ring Mushroom is based on, and gets her name from "fairy rings", which are a ring of smaller mushrooms that, in numerous European folklore, are often seen as hazardous and dangerous areas. Specifically, Fairy Ring Mushroom is based on the mushrooms that form the ring such as the mushroom of the same name (Marasmius oreades), also known as the Scotch bonnet or fairy ring champignon.
The "Fairy" part of her name alludes to how she presumably transforms zombies using magic, while the "Ring" may allude to how she attacks in a 3x3 "ring" around herself.
Plant Food effect
Fairy Ring Mushroom's Plant Food ability is called Mass-Shroom, which deals heavy damage to all zombies in a 5x5 area around herself.