In Year 15 of Konoha, Xingzhuo used the Flying Thunder God almost every day to travel to the Land of Fire's border. There, he transformed natural materials into Explosive Tags, then changed them back again for future use.
That same year, two-year-old Tsunade had just become aware of the world around her. She abandoned her gambling-addicted grandfather and liked to cling to the handsome and dashing Hyuga Hoshishaku and Senju Yuto.
As it happened, that was also the year Xingzhuo finally collected enough fragments to assemble a full S-rank ninja through his card draws, gaining the third S-rank ninja of his life: "Tsunade."
At the same time, the S-rank ninja in the permanent card pool was replaced with "Mangekyo Sharingan · Uchiha Sasuke," and a new SS-rank ninja, "Uchiha Obito," was added. The latter's drop rate was pitifully low, though; both times, Xingzhuo had only gotten the guaranteed five fragments.
Tsunade's abilities needed no explanation. She was a true master of Medical Ninjutsu, and her strength was undoubtedly at Kage Level.
Xingzhuo contributed all the Medical Ninjutsu he gained from Tsunade to the village. As compensation, he decided to take her as his student and teach her personally, hoping she could stand on "his" shoulders and take the path of Medical Ninjutsu even further.
Of course, as his first student, Tsunade would certainly learn the Natural Energy Series techniques from Xingzhuo in the future, rather than using the inferior Yin Seal to maintain a false appearance of youth.
Speaking of the Natural Energy Sealing Technique, Xingzhuo was actually pretty slow on the uptake.
It wasn't until Year 15 of Konoha, when he was fifteen, that the Natural Energy sealed inside him underwent a qualitative change due to its increasing quantity, and he entered Sage Mode naturally.
However, his Sage Mode