I will explain what has happened in short:-

At Chapter 27 end, Yukan was 13 and at the start of Chapter 28, he was 18.

In Chapter 28, After the line,

"It is indeed going as I planned." Zetsu giggled and went under the ground.

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A flashback of two years started, meaning back to when Yukan was 15 , to explain things, like, who died, why they died, who is Kozuki, the boy who stood behind Sakumo, why did he call Yukan "Nii-San" and all that.

If there is still any confusion, comment and ask me.

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And, I have selected the second template. I know that Kenshin Himura also had sword skills, but the other character has some more advantages other than sword skills.

The character is from a manga called, "Claymore". People who have nothing to do may read the manga. It has a little similarity to AOT.