Celine is a unique alien lifeform originally given to the main protagonist Rito Yuuki as a birthday gift from the Devilukean princess Lala. She began her life as a rare, multi-story sentient sunflower with a mouth full of sharp teeth and vine-like appendages, which resided in the Yuuki family's backyard. Despite her initially monstrous appearance, Rito cared for her deeply, considering her a member of his family. When the giant plant appeared to be dying from a disease, Rito went to great lengths to find a cure. It was eventually revealed that the plant was not dying but germinating, and from it emerged a single seed that burst open to reveal Celine in her current iconic form: a small humanoid infant with blue eyes, light green hair, and a small flower perpetually blooming on her head.

Celine’s personality is that of a typical human toddler, full of curiosity and boundless energy. She is often seen running around the Yuuki household and town, getting into mischief with a completely innocent demeanor. She is fascinated by machines, Lala's often malfunctioning inventions, and Momo's plants, frequently interacting with them without understanding the consequences, which leads to a variety of chaotic and comedic situations. As an infant, she has an instinctual tendency to leap onto the chests of women, seeking to breastfeed, which consistently leads to awkward and perverted accidents for those around her. When she is not causing trouble, Celine enjoys typical activities such as playing video games, watching magical girl shows, gardening, and eating large amounts of ramen. Although she can be fussy and impatient like any child, she also shows an understanding of discipline and often looks remorseful with a finger in her mouth when being scolded.

Celine has a range of notable abilities due to her unique plant-based physiology, which remains a mystery even to experts like Momo. Her most frequently demonstrated ability is the release of a special pheromone pollen from the flower on her head. This only occurs when she is drunk, which happens after she consumes cola. Anyone who inhales this pollen will grow a small flower on their own head and temporarily fall madly in love with Rito, a side effect caused by Celine’s own deep affection for him. The effects of the pollen typically wear off within half an hour. Another significant ability is her transformation during a full moon. On nights when the moon is full, Celine temporarily grows into an older, more mature girl. In this form, she is calmer, can speak in complete sentences, and retains her loving and playful nature. She reverts back to her infant form when the night is over.

Her role in the story is primarily that of an adorable and chaotic force of nature, serving as a recurring source of comedy and accidental romantic entanglement. She is a beloved member of the Yuuki household and often acts as a catalyst for perverted situations, either by jumping on someone or by releasing her love pollen on unsuspecting victims. Due to her closeness and resemblance to Rito, she is frequently mistaken for his daughter by other characters.

Celine’s key relationships are all familial in nature. She views Rito as her father, given the immense care and devotion he has shown her since her time as a large sunflower. This bond is so strong that the pollen she releases targets love specifically for him. She shares a sisterly bond with Momo, who is one of the few people who can communicate with her directly due to Momo’s own ability to speak with plants. Momo often teaches Celine right from wrong and, on occasion, uses Celine’s pollen as a tool to further her own "Harem Plan". Mikan Yuuki also shares a very close and motherly relationship with Celine, as she is often seen playing with her and caring for her. Mikan was also the one who gave her the name Celine. Kotegawa Yui also has a significant bond with Celine, as Celine is drawn to her. This has led to Yui being mistaken for Celine’s mother and the two are frequently depicted together, with Yui often daydreaming about adopting Celine.