Ephemer is a Keyblade wielder first introduced in Kingdom Hearts χ. He is a member of the same Union as Skuld, though which Union this is is directly dependent on the Union that the player is a member of.
Ephemer constantly seeks to discover the truth behind the gathering of Lux and the structure of the world. He displays a high level of curiosity. Despite the Book of Prophecies being highly restricted, he desires to have knowledge and truth, even talking to one of the Foretellers, most noticeably Ava, as both have a casual conversation by the fountain in Daybreak Town.
Centuries after his physical death, Ephemer was still dutiful of his duties as a Keyblade wielder and new Union Leader, as his heart remained in the Keyblade Graveyard and assisting the new generation of Keyblade wielders in fighting against the 13 Seekers of Darkness.
Ephemer has curly, silver hair and blue eyes. He wears a white shirt, black vest, dark grey pants, and black boots. A red scarf is wrapped around Ephemer's neck.
Ephemer is a member of a different Union to the Player. When he first meets them, he reveals that the Lux the player is tasked with collecting from the various worlds truly belongs to Daybreak Town. He also claims that the worlds the player has visited up to this point are actually illusions created by the tomes of the Book of Prophecies that each Foreteller possesses. After the player tells Ephemer about a strange dream they had which included the Foretellers and another mysterious figure, the Keyblade wielder decides to team up with them and investigate this matter.
Together, Ephemer and the Player dispatch several Heartless as they make their way towards the Foretellers' headquarters. The two ultimately decide to abandon their investigation, and soon afterwards, Ephemer mysteriously disappears. Some time later, the Player has a dream about Ephemer in which the Keyblade wielder asks Foreteller Ava to divulge the secrets of the Book of Prophecies. Ava refuses.
The next day, Ephemer reunites with the Player in their dreams. The two briefly resume their earlier investigation. When the Player awakens, they learn in a conversation with Foreteller Ava that Ephemer may be in another dimension and is possibly using dreams to communicate as he closes in on a hidden truth.
When the Player meets an enigmatic Keyblade wielder named Skuld, she reveals that Ephemer is from her Union and seems to have changed. Skuld claims Ephemer is also appearing to her in her dreams; in one, she reveals, he tells her the world is nearing its end.
True enough, he learned the truth much more faster than the other Foretellers. Ava meets Ephemer and gives him a task to go to the outside world. When the time comes, he and the other Keyblade wielders who are not involved in the war have to recreate the world after the original world meets its end. Thus, due to their role in guiding the Dandelions, Ephemer and Skuld were not involved in the war and their hearts were not covered in Darkness. While working towards restoring Keyblade wielders, the pair met another wielder, also working for Master Ava, who was later revealed to be Ventus.
At an undetermined period following the Keyblade War, Ephemer's physical body perished. However, part of him remained in the Keyblade Graveyard, likely due to his connection to the place. Shortly before the confrontation between the Seven Guardians of Light and the Thirteen Seekers of Darkness, he appeared in a vision to Sora, assisting him alongside other ancient Keyblade wielders in a battle against a colossal Demon Tide, the former taking the form of the Keyblade each wielded in the past. After the battle, the Keyblades converged and disappeared in a burst of light.
As a Keyblade wielder, he can wield a Keyblade in combat.
As the spirit within the Keyblade Graveyard, Ephemer was capable of manifesting in Sora's vision and command the fallen Keyblades to assist the group in battle against the whirlpool of Heartless.