Ketsuno Seimei (結野 晴明) is the current head of the Ketsuno clan. He is a genius and is considered to be the strongest onmyoji in the clan's entire history. He is also the brother of Ketsuno Crystel.
During his childhood he was considered a prodigy, showing great talents within the onmyoji arts. He tried to befriend Shirino Douman, but due to an old clan feud their friendship didn't last long. The Ketsuno clan and the Shirino clan had been in a savage struggle for political ever since the Heian period.
At an older age, having become the head of the Ketsuno clan, Ketsuno Seimei would try to bring peace between the two clans by proposing a peace accord to the then head of the Shirino clan; Shirino Douman. He said he would agree to the accord, on one condition: Ketsuno Crystel's hand in marriage. For the future of the clans, both Seimei and Crystel agreed to this. However, as days passed by Seimei would notice that Crystel was unhappy in her marriage, so he forcefully took her away from Douman, arranged a divorce and broke the peace accord. This caused the clan rivalry to become even stronger than before.
He has short, sandy brown hair with a ponytail and dark purple eyes. He is usually seen wearing the Ketsuno clan's onmyoji uniform.
Seimei is strict and hard-working, but also cares a lot for and worries about his sister Crystel.
Ketsuno Seimei is currently working to protect the Bakufu and Edo by foretelling disasters and ill omens. For this purpose he has stationed over 1000 of his shikigami all around Edo as his "eyes". It's his duty to process the information the shikigami send, use it to determine which direction the nation is heading in and how the Bakufu should act.
He leads his tribe together with Shirino Douman to fight with the amanto during their invasion. Since they come relatively late during the first battle, they take up the responsibility to guard Kabukicho district during the nighttime.
- Based on Abe no Seimei who was a famous diviner/onmyoji in the Heian Period, believed to have been skilled in manipulating spirits/shikigami. Myths involve him in magical battles with his rival Ashiya Doma who wanted to take his position.
- In Gintama, Seimei summons a fox-like shikigami called Kuzunoha who is based on the actual Kuzunoha, a fox/kitsune in folklore who is the mother of Abe no Seimei.