Sei-chan passed away (of old age, probably). Her Fudou was dismantled according to her will, and a new Saniwa came to take over her Honmaru, also according to her will. The new Saniwa manifests a new Fudou as one of the first things they do as Saniwa.

This Mukotsu met Fudou when Fudou was already Kiwame'd so he didn't know Fudou was a mess as Ubu. So, instead of needing to look after a Ubu Fudou, Mukotsu looked after Sei-chan, who called him "Niisan" (from the "Ni" in Ningen) and looked like a harmless cute little animal in Mukotsu's eyes. Mukotsu adores Sei-chan like a little sister (and by extension, Fudou is like his little brother).

I am the yari wielded by my partner, the Oni Musashi, I am merely a weapon. But as Touken Danshi, I held feelings unknown to a mere weapon. Feelings such as nostalgia in seeing a face unfamiliar yet familiar to me.
It was the first time I saw Fudou Yukimitsu’s face, and yet I was relieved to see him. Strong and capable, being trusted the most by the master. The said master, however, was frail. Small and frail, like the brothers of my partner. She looked like she would fall from the slightest push. But contrary to my first impressions, this master was not frail. She held an unwavering will and resilience to fight many battles alongside us for many years, right until her short human life ended. And the one supporting this small yet strong master was always Fudou Yukimitsu. Even as a mere weapon, I was proud of the two.

“To have Fudou Yukimitsu dismantled after passing, and to keep the citadel running under a new Saniwa.”
Those were the wishes of the little master. Nobody held an objection, for Fudou Yukimitsu was not able to bear the passing of his loved one, and this citadel contains many memories and experience over the decades of the little master’s service as Saniwa. Currently, we are on standby as Kasen Kanesada and the Kudagitsune help the new master get used to working as a Saniwa.
It was one week after the new Saniwa came over. We finally meet the new master in person, and alongside the greenhorn was…
“Hic!”
Long violet hair, face flushed red, and a cup in one hand. It was a face familiar yet unfamiliar to me.
“What the fuck is this.”
I am not clueless as to what this is. This is Fudou Yukimitsu, the one that left with the little master—my former master. The Fudou Yukimitsu I know is not… *this*. Blood rushed to my head and my body heated up. *I cannot accept this.*
“Calm down, Ningen Mukotsu!” Kasen Kanesada pulled me away from *that* and the new Saniwa. Kasen, the first sword to have met our former master. He should know better than anyone just how much of a strong bond our former master and Fudou Yukimitsu had. How could you let this happen?
“I understand you have your doubts, but this is what our new master decided.” Our new master, who is unable to even show us their face or speak directly with us. Even though our former master, smaller, was able to see eye to eye with every one of us. Ah, to realize how great my little master was only after her passing. This crushing feeling, is it grief? Was this what my partner had to bear when he learnt of his brothers’ death?

Oh how I wish to see their faces one more time…

. . .

I am the yari wielded by my partner, the Oni Musashi, I am merely a weapon. And as a mere weapon, I will fight… and beat some sense into this new Fudou Yukimitsu!
“Huff… huff… What the fuck is wrong with you…! First you were staring daggers at me, now you’re trying to break me or what?!”
“Break you? This is just sparring using mock weapons, you won’t break. But if you have enough energy to speak, you don’t need a break, do you!”

As Touken Danshi with human-like feelings, I will hold my former master’s wishes close to heart. This citadel that she and her beloved cherished will be maintained, and these two greenhorns will be trained so that they can help maintain our citadel.

So rest in peace, my two little kin.