Kazusa is a researcher in Akademeia who wants to conduct his own, rather creepy, investigation on Class Zero, with a sidequest centered around that. You will first encounter him in Free Time 2.2, where you will find him in the Armory. There will be an event there, then one more where he will take you to his secret lab, hidden behind a bookshelf on the left side of Crystarium (when facing the Rubicus after first entering the area). See, we told you he was creepy!

That secret lab is where you will find Kazusa for the rest of the sidequest. Note that similar to Emina, Kazusa will only be found when the hours left are between 2 and 6 for any given day. You will need to visit him with specific characters, which will trigger a short cutscene. At first, there will be a total of seven scenes that can be seen and you can revisit Kazusa on a different day with the same character to view it again. The characters you need to visit him with are below:

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Kazusa’s Lab (once you meet him for the first time) can be found behind a bookcase in the Crystarium (left), but he’s only there when there’s less than six hours on any given day (right).

Upon viewing all scenes, you will need to wait until Chapter 7 to continue the sidequest (even on subsequent playthroughs). Talk to Kazusa, then go to Central Command and speak with Naghi to get an item. Once you get that, go and give it to Kazusa in his lab, then speak with Naghi in the Crystarium to do his task to find Kurasame’s Knowing Tag. Return to Kazusa and you’ll be able to view the scenes below, similar to the ones above. After finishing all of them, you can examine the yellow capsule in his lab to finish the sidequest.