Phoenix is the immortal bird of fire at the center of the anime of the same name. It is a supernatural being that has lived endlessly from the distant past into the far future, watching over the history of humanity and the Earth. In the stories, the Phoenix is a symbol of eternal life: it is said that anyone who drinks its blood gains everlasting life, and a single stroke of its feather can heal the injured or the seriously ill. Because of these powers, countless human characters spend their lives seeking the bird, chasing after it, and fighting one another for the chance to capture it. The Phoenix is also known by several names, including the immortal bird, the bird of fire, and the fenghuang, reflecting its status as a legendary and quasi-divine creature rather than an ordinary animal.

As a presence in the series, the Phoenix acts primarily as a witness and chronicler. It appears across radically different eras, from prehistoric times to humanity's distant future, and its appearances connect the otherwise separate stories into one continuous drama. It transcends time and space, can appear in more than one place or form at once, and is able to enter human dreams, where it reveals visions of past lives and the cycle of rebirth. Rather than having a personal goal of its own, the Phoenix is motivated by its role as an eternal observer of human struggle. It quietly watches humanity's endless pursuit of power, justice, and freedom, and it appears to individuals at pivotal moments to guide them, test them, or reveal deeper truths about life and death.

Humans in the series regard the Phoenix's blood as the ultimate prize because it promises immortality, but the Phoenix itself embodies a more complex view of eternity. In the worldview of the series, the natural path of life is reincarnation, the endless cycle of being born, dying, and being reborn, while the immortality humans crave is either impossible to truly obtain or becomes a terrible burden for those who reach it. The Phoenix therefore stands in contrast to the human characters who seek it: they are driven by fear of death, ambition, greed, or attachment, while the Phoenix is serene, detached, and enduring, offering healing and insight but also confronting mortals with the consequences of their desires.

The Phoenix's relationship with the human characters of each era is usually brief but profound. It does not form lasting bonds in the way human characters do, and it does not undergo personal development over the course of the series, because it is an unchanging, eternal constant against which human lives play out. Instead, it interacts with different seekers, artists, rulers, and wanderers, rewarding compassion, condemning arrogance, and reminding each generation that life, death, and rebirth are all part of the same unbroken cycle. Its abilities include immortality, flight, fiery brilliance, healing through its feathers, life-giving blood, the power to move across time and space, the creation of multiple forms, and the ability to show mortals their past incarnations. Through these powers, the Phoenix serves as both the object of humanity's greatest desire and the calm, unshakable witness to everything that desire brings about.