Dan Katō was the uncle of Shizune and the lover of Tsunade Senju.
He died fighting a group of Ame-nin when his squad retrieved some information from behind enemy lines.
In truth Dan´s unit was betrayed by Furofuki who had planned to have Tsunade die at the hands of the Ame-nin.
Dan´s grave was one of those robbed by Kabuto Yakushi for use in Edo Tensei.