Apparently, Kusaka uses the King's Seal to come back to life for revenge, but in the end is killed by Hitsugaya in a final rematch. He also appears in Bleach: Fade to Black, in which he loses his memories of Ichigo and Rukia, along with the other Soul Reapers.

Why is hitsugaya a zombie?

When Mayuri appears behind him and notes his body moves exactly as it did when he was alive, Hitsugaya slashes at him, only for Mayuri to dodge with Shunpo while deducing that Hitsugaya was turned into a zombie before he died because he does not have a consciousness like Bambietta does.

Is Toshiro hitsugaya the weakest captain?

#13: Hitsugaya

I consider Hitsugaya the weakest of the captains simply because he is the youngest - when Bleach begins, his bankai isn't fully mature yet, and it tends to break whenever he uses it. If Kyoraku is right, Hitsugaya is going to be the strongest someday.