Shirogarasu was also surprisingly popular and stayed in the top 10 in character polls for a long time, despite his short tenure as a character and very minor role in the overarching story. He was still 18th in the final poll... when he'd been dead and unmentioned for over 30 volumes.
Harsher in Hindsight: Yuan refers to Hishigi as "an incredibly efficient executioner" when explaining why getting any information out of him is going to be difficult. We find out later that Hishigi has quite literally acted as the Former Crimson King's executioner, and he killed Yuan's mother on the king's orders.
Magnificent Bastard: The Former Crimson King, certainly. Yukimura also definitely qualifies, and Nobunaga might.
Specifically, Yukimura's tactical and strategic skills are such that Ieyasu would rather bribe him than have him as an enemy. At the end of the series, he successfully anticipates the Former Crimson King's plan and has already had Okuni rally together the combined forces of the Tokugawa, the Sanada, and the Date to contain the King's blood soldiers and keep them from rampaging out into the human world.
Moe: Shinrei was ridiculously adorable as a child. It's a serious contrast to Hotaru around the same age; he was incredibly angry and already scarred by the time he was six. Shinrei had the world handed to him — Hotaru had to fight just to survive.
By the time Tokito finds her father (Fubuki) after regaining her memories, he is near death. She begs the Former Crimson King to save him. The Former Crimson King attempts to kill her instead; but Fubuki (already dying and likely beyond saving) takes the fall.
He manipulates Kyoshiro (through Chinmei) into killing Sakuya's brother, and arranges for Sakuya to watch. Made worse since Nozomu was also Yuya's adoptive brother, and his death was her Cynicism Catalyst.
His We Have Reserves speech on not curing the false Mibu is what caused Muramasa to abandon him.
Slow-Paced Beginning: The first few volumes read a bit like an attempt at cashing in on the popularity of Rurouni Kenshin. The beginning is still enjoyable, especially once SDK finds its own uniquely odd voice, but the narrative isn't anything mindblowing. The real plot with the Mibu hits in Volume 10, and from then on, we have something extraordinary on our hands.
Too Cool to Live: Muramasa, Taihaku, Fubuki and Hishigi. One can also make an argument for Kyosaburo, who created the Mumyo Jinpu Ryu and passed it on to Muramasa and therefore, by extension, to Kyo.
Uncanny Valley: Kyo's Crimson Eyes look strange and intimidating, but they fit with his overall aesthetic and personality. The Former Crimson King's and Kyoshiro's variants just don't look right, and the Mibu Reconstruction Plan's child experiments are even worse.
The Woobie: Tons of characters, like Yukimura. Pretty much any character who gets any attention from Kamijyo has some Woobie elements.
Stoic Woobie: Hishigi, good god. He is in constant, excruciating pain from the Devil Eyes and the Death Disease. He has watched countless friends die horribly while he was helpless to save them. But he keeps fighting for Fubuki, because Fubuki is all he has left. And not once does he complain about any of this. He even tells Fubuki not to feel sorry for him while he's dying in Fubuki's arms, because Hishigi chose his own path and Fubuki is not allowed to blame himself.
Woobie, Destroyer of Worlds: One can make this argument for The Former Crimson King. Kyoichiro was the first of the Crimson Cross Knights, four Battle Dolls that were the closest thing the True Mibu ever created to their own power. When the True Mibu died out, Kyoichiro tried to keep their world together and rule with benevolence, but millennia of watching people hurt each other jaded him into a breakdown that eventually led to him losing all faith in both the Mibu and humanity.