Once you reach the Senpou Temple for the first time, head to the right after exiting the building to find Kotaro slumped up against a tree. Speaking to him will reveal that he’s looking for a pure white flower, the White Pinwheel, that you’ll find in this area.
Handing Over the Pinwheel
Once you obtain the White Pinwheel or the Red and White Pinwheel, return to Kotaro and you’ll be given a choice of which one to give to him. Giving him the Red and White Pinwheel will give you a choice to send him to Anayama or Doujun provided you’ve advanced their questlines, doing so will end Kotaro’s quest however.
Should you choose to give him the White Pinwheel, he’ll ask you to spirit him away.
After Obtaining the Divine Abduction Tool
Once you obtain the Divine Abuduction Tool in Senpou Temple, return to Kotaro and use it on him, transporting him to the Hall of Illusions.
After Obtaining the Mortal Blade
Once you’ve finished the Senpou Temple area and obtained the Mortal Blade, return to the Hall of Illusions to find Kotaro at the entrance. Upon speaking to him, you will see him happy and obtain Taro’s Persimmon, thus concluding his quest.