Persona 5 Royal offers a wealth of new features and content in comparison to the base game, and one of these new additions is Will Seeds. There are three of these special items hidden within each palace that you encounter in the game and collecting all three will give you a new accessory.
Additionally, each time you pick up a Will Seed, your entire party regains a little SP. If you miss any Will Seeds during a palace run, you can buy them later from Jose's Shop, but they'll cost you a small fortune. Here's a full guide on where to find all of the Will Seeds in Kaneshiro's palace.
Will Seeds are always found behind overgrown doors, and one of your teammates will notify you when you are close to one.
Red Gluttony Seed Location
Once you reach the room with the large vault door/giant partition on the lower area (in The Banker Passageway), take a right from the vault and head down the passageway until your hand glows to show a grapple point is nearby.
Once you've grappled up to the next level, head to the northernmost room to find the Will Seed.
Green Gluttony Seed Location
Once you reach the Laundering Office, head up the linear pathway and take a left and then follow the path all the way down to the most southern room. In this room, there is a vent next to the doorway that you can crawl through.
Crawl through and then move to the end of this ledge and jump down. On the next ledge, go to the southern end and jump down.
On the next ledge, jump onto the platform next to it, then climb up to the next platform from there. From here, you can grapple straight to the next Will Seed room.
Blue Gluttony Seed Location
Once you are in the large circular lock room, you'll be inputting various codes to move into the center of the room. After you have defeated the mini-boss, interact with the panel that was behind it and choose "Press the right button." Then interact with the panel and press the button again.
Move into the newly-opened area and interact with the panel here. Choose "Press the right button.", before interacting with the panel again and pressing the button again.
This will open up a new hallway with a mini-boss you need to defeat in order to get to the final Will Seed. The Guard Dog of Hades is weak to Ice, so maximize on hitting its weakness to take it down easily.
Once you've defeated the Guard Dog of Hades, head through the door it was guarding and enter the Will Seed room to get the Blue Gluttony Seed. Now that you have all three Will Seeds, they will fuse together to create the Crystal of Gluttony accessory.