Lulu, also known as Kaeru no Ruru or Ruru, serves as a guardian angel to Gorō Mutsumi. She embodies the reincarnated spirit of a frog that perished from hypothermia during an unexpectedly cold summer day. This occurred after Gorō and a young girl rescued and nurtured her, only for her to escape accidentally and freeze to death. The trauma left her with cryophobia and frigophobia—persistent fears of cold and cold objects.

As the 12th and youngest guardian angel in her group, she bears chartreuse as her symbolic color, a vivid fusion of yellow and green. Following her death, she traveled to the spirit realm but lost her way in the void before returning as an angel.

Lulu displays a childlike personality, craving attention and physical closeness with her master. Though occasionally spoiled or bratty, she harbors no malice. Her emotions shift rapidly: quick to forgive yet prone to anger when contradicted. She often hugs Gorō, misinterpreting her paternal attachment as romantic affection.

Among fellow angels, she shares a mostly harmonious bond with Momo, whom she teases but seeks for comfort. With Nana, conflicts arise easily but resolve just as swiftly over shared activities or meals. Initially distrusting Daisuke alongside Nana, Lulu later adjusted to his presence.

Notable details include her green hair and eyes, a height of 115 cm (3'9"), favorite color (green), preferred food (spaghetti), birthday (June 21), and star sign (Gemini or Cancer, varying by source). She enjoys climbing as a hobby.