In "Hell Realm Mandala: Heian-kyo," although he had gone missing, he conveyed the ritual of the Imperial Grail War to Fujiwara no Michinaga through a letter sent from wherever he had disappeared to, causing it to be held. In the latter half, whether he changed his mind when Alter Ego Limbo revealed its true nature and manifested a subspecies Fantasy Tree, or whether Limbo had merely been using Seimei's name all along, he made contact with the protagonist and their companions through letters. While supporting the resolution of the situation, when Limbo failed to become a Beast, he even sent him a letter as if to provoke him. After the incident was resolved, he appeared in his true form before Ashiya Douman, who had regained his sanity, and conversed with him.
The person himself never appears in the story, and since he communicated through letters that floated up and wrote themselves in midair, his actual personality is unknown. However, he does not seem to have a very upright character, as he says things like "just timing" that are clearly anachronistic, and while he is proactive about resolving the situation itself, he uses people without much consideration for their feelings.
He displays superhuman ability, such as erecting a barrier over all of Heian-kyo to prevent powerful demons above a certain level from entering, and using that barrier to grasp everything occurring within Heian-kyo. Also, according to Kintoki's testimony, he can apparently wield a technique called "Gojoshinkan."
He left behind information about the future obtained through clairvoyance, as well as knowledge of the Grail War, as a legacy to his descendants.
However, it seems he left behind only incomplete information about the Holy Grail and the Grail War, as if to say, "Figure out the rest for yourself."
*This is a setting in which he has been implemented and is manifesting at Chaldea (as an unimplemented character, various details are merely an image).
A representative onmyoji of the Heian period. Uses "watashi" as his first-person pronoun.
A man of mysterious wonder and dubious credibility.
Though not yet implemented, according to Tamamo-no-Mae, his appearance is praised as "a peerless beauty with sharp, narrow eyes," but regarding everything else, he is described as a "dark ikemen," "dyed to the soul in the colors of the Nether Realm," and "doesn't have the bone structure of a Japanese person."
𓇠𓇠𓇠𓇠𓇠𓇠𓇠𓇠𓇠𓇠𓇠
In Fate/EXTRA, he first appears through the mention of Tamamo-no-Mae, whom he exorcised. She disparages him relentlessly as a "dark ikemen" and "dyed to the soul in the colors of the Nether Realm," holding him in bitter contempt. His appearance is that of a peerless beauty with sharp, narrow eyes, an ikemen (a monster [“mon”ster] posing as an “i”ke “men”) who doesn't have the bone structure of a Japanese person.
In Fate/Grand Order, he is first mentioned in Part 1.5, "Shimousa."
A rival onmyoji who used his name assessed him as a generally good onmyoji, and it is said that the righteous path of fighting to live, like Chaldea's Master or Musashi, resembles him.
Senji Muramasa lists him as a formidable Caster on par with "the invincible swordsman Jubei" and "the ninja daimyo Hanzō."
For a long time he did not appear, and impressions among those connected to him varied, leaving his character unclear (Tamamo and others held a hostile stance).
Only Muramasa, from a different era, clearly evaluated his abilities.
He appears in Part 2, Chapter 5.5, "Heian-kyo," in the form of communication via flying letters, revealing a glimpse of his character.
He excels at the power of "seeing," such as observing events in Shimousa, and despite being from the Heian period, he possesses knowledge of modern terms, foreign matters like Zeus and Caesar, and awareness of Chaldea.
According to Kintoki, he looks young but is an old man, with an aura similar to Holmes.
In Douman's Valentine's scene, he seems to overlap the Master with Seimei's image.
As Murasaki Shikibu says, his personality reveals a good-for-nothing nature that sweeps those around him along.
In Fate/Samurai Remnant, he left information about the future of the Grail War seen through his clairvoyance to his descendants, and based on that, Tsuchimikado Yasuhiro created the "Eigetsu"—in a sense, he is the root cause.
He left only incomplete information, as if to say, "Figure out the rest for yourselves."