The Medium, also known as Xiuan, is the young human girl that leads the Vessels, a species of humanoid beings that live in the Misty Ravine. She guides the Vessels and helps keep their town secure from the Hollows, beings that essentially look like humanoid spirits that also dwell in the Misty Ravine. She is the only human part of the group of Vessels, with the innate ability to "hear Yggdrasil's voice". The Vessels rely on the Medium, who is able to relay Yggdrasil's guidance.
This ability to keep the Yggdrasil in check eventually puts her in an important role in the story, as she is actually an embodiment of the Titan's Soul. This results in her getting kidnapped by Logre and then Baldur in order to use her to activate the Yggdrasil and awaken the Heavenbringer. It is only after the Heavenbringer's defeat that the Medium is finally rescued and returned to safety in the Vessel village.
In the postgame, 2 in-game weeks after the Heavenbringer's defeat, a Vessel will appear in Tharsis and inform the player of the Medium's increasingly tired condition. This signifies that the player can re-visit the Medium. If they opt to watch over the Medium as she sleeps then, the party will do battle with the Heavenbringer again. Following that, the Medium will return to normal, but 2 weeks later the event will recur.
- It's possible that The Medium is some form of Yggdrasil Core. Though she bears no resemblance to other cores, her deep connection with Yggdrasil along with the fact that it's not active without her fusing with it make this a possibility.
- Being an important component of the Imperial Yggdrasil Tree implies the possibility that Xiuan hailed from the era before the Apocalypse, maybe even as a member of the Yggdrasil Project.