Melancholy is a central antagonist introduced in the second season of Phi Brain - Puzzle of God, where she appears as a member of the enigmatic Orpheus Order. She continues to play a significant role in the third season, transitioning into a Master Brain serving under the entity known as Enigma.

Physically, Melancholy is notably short in stature, which often leads others to mistakenly believe she is a young child, a misconception she firmly denies. Her appearance is characterized by long orange hair and blue eyes. Her attire typically consists of a dark blue blazer worn over a white shirt with a matching necktie, a dark blue skirt, white boots, and a dark blue hat adorned with a red rose.

Melancholy's background is deeply rooted in tragedy. She was born into a wealthy and noble family, displaying exceptional talent for creating and solving puzzles from a very young age, hinting at her latent Phi Brain potential. Her parents were killed in a car accident while she was still young, leaving her in the care of her uncle. This remaining family member also met an untimely end, dying in a Fool's Puzzle constructed by Freecell, a fact Freecell himself remained unaware of for a long time. With no other guardians, Melancholy felt she had no choice but to join the Master Brain and, by extension, the Orpheus Order.

On the surface, her personality appears childish, arrogant, and selfish, giving off the aura of a spoiled and petulant young girl. However, this behavior is eventually revealed to be a carefully constructed act, a facade she adopted to hide her true, more vulnerable nature. Beneath this mask lies a profound sense of loneliness and unresolved trauma stemming from the loss of her family. Her grudge against Freecell for her uncle's death is a primary driver of her actions, though it masks a more complex desire for connection.

Throughout the second season, Melancholy serves as an active antagonist. Her puzzle-solving skills are first showcased in a sliding puzzle match against Cubic, where she is aided by her Orpheus Ring. Following a defeat, she later challenges Rook to a deadly collapsing maze puzzle. One of her most pivotal acts during this period is her calculated betrayal of Mizerka, whom she had pretended to regard as an older sister figure. Melancholy cruelly reveals that her affectionate "little sister" persona was entirely an act, confessing she found Mizerka inferior and merely tolerated her presence. This emotional betrayal contributes directly to Mizerka's mental breakdown. Later, she allies herself with Klondike and Whist, only to have her Orpheus Ring shatter from sheer terror when confronted by the overwhelming power of Freecell's own ring.

Following the fall of the Orpheus Order, Melancholy resurfaces in the third season with a singular goal: to exact revenge on Freecell for her uncle's death. She becomes a Master Brain working under Enigma and lures Kaito, Gammon, Nonoha, and Freecell into a four-player Fool's Puzzle designed for only one team to survive. However, her plan backfires when Enigma forces her to be the fourth player, and Kaito's ingenuity allows everyone to escape unharmed. After the puzzle, she invites the group to her mansion, where, during a blackout, she finally explains her tragic past and the source of her grudge against Freecell.

Her emotional turmoil comes to a head when she finds Kaito, Gammon, and Freecell playing with puzzles in a room she used for her Master Brain training. In a fit of rage and sorrow, she gathers all the puzzles, which she admits were tools to heighten her Phi Brain abilities, and burns them while Freecell desperately pleads with her to stop. The next day, she is later seen tearfully collecting the ashes, leading Kaito to realize that she had been creating puzzles out of loneliness. While she ultimately and bluntly rejects their offer of friendship, Kaito and Freecell leave behind a crossword puzzle with a hidden message promising they will meet again.

As the third season progresses, Melancholy undergoes significant development, shifting from a vengeful antagonist to a reluctant ally. After Orpheus takes control of Jin, she is shown a vision of a ruined future, which prompts her to provide Kaito and his friends with transportation aboard her private yacht to help them on their mission. Her softening nature is further evidenced by her willingness to help Nonoha bake cookies and her participation in a girls' slumber party with Ana and Raetsel. Most notably, she is heard talking fondly about Freecell in her sleep, suggesting that her feelings of hatred have evolved into something more complex and potentially romantic after he was freed from the influence of his Orpheus Ring.

Key relationships define her character arc. Her relationship with Freecell is the most central. It is marked by intense fear in the second season, born from a grudge he does not know he carries. By the third season, this transforms into a desire for revenge, which Kaito and Nonoha suspect is actually a subconscious excuse to see Freecell again. Freecell himself consistently seeks her forgiveness and friendship. In contrast, her relationship with Mizerka was built on deception; she manipulated Mizerka's genuine affection for her own purposes, leading to Mizerka's downfall. Her dynamic with Kaito and his friends evolves from antagonism to a tentative, albeit refused, acceptance of their concern and friendship.

As a Child of the Phi Brain and a Master Brain, Melancholy possesses exceptional puzzle-solving abilities, which were evident from her early childhood. Her skills are often augmented by an Orpheus Ring, a powerful artifact that enhances the user's puzzle-solving capabilities. Her childish and arrogant demeanor often serves as a smokescreen for her sharp intellect and cunning strategic mind.