The main fun of Digimon Story Cyber Sleuth is raising up a small army of Digimon. Unlike the Pokémon games, where you're encouraged to focus on one core team, Cyber Sleuth wants you to experiment with Digivolving, De-Digivolving, and swapping out your party members for the critters in your farm.
There is one obstacle to your complete Digi-freedom, however. Party Memory is a stat that limits how many Digimon you can take with you. The way it works is that every Digimon has a certain amount of memory they take up. This number increases when they Digivolve. You, meanwhile, have a maximum Party Memory. Once that's exceeded, you can no longer add to your party or even Digivolve a current member. You start with 20 Party Memory, which is barely enough to field a team of Rookie-level Digimon. So how do you get more?
How To Get Memory UP
Memory UP and Memory UP DX are the only items in the game that can increase your Party Memory. Unfortunately, you can't buy them in a shop or craft them on the farm. There are a limited number of Memory UPs available in the Cyber Sleuth story, and they're spread out throughout the various chapters. You'll typically receive one after beating a major boss fight, though there are a couple hidden in chests as well.
Memory UP Location Guide
These locations come from the wonderful folks at Gamers Heroes and GameFAQs, who did the legwork in discovering them. We're going to break them down into three categories based on how you get them: boss battles, side quests, and chests.
Again, the big boss fights of each chapter should reward you with a Memory UP. The bosses confirmed to give them are:
-Devimon and Meramon (Gives two Memory UP)
-Train Station Boss
-Yamashina's Memory Boss
-Crusadermon (Fought twice, in chapter 17 & 18)
You'll get chances throughout the story to do extra cases. These are worth taking, as they often reward rare items like Memory UPs. The cases known to give Memory UP are:
-Nightmare Eater in Chapter 4
-Case La Missel in Chapter 9
-Shinjuku Food Tour Case in Chapter 13
-A Pet Digimon in Chapter 15 (gives two for two different fights)
-Yuuko And Yuugo in Chapter 15
-Ghost Cat Building in Chapter 16
-Dr Datamon's Medicine Trial Case in Chapter 17
-Boy A in Chapter 19
-Rina Shinomiya's Challenge in Chapter 19
-Comeback Show in Chapter 19
These ones will be harder, as they require you to find the chest in question. The general chest locations are:
-Kowloon Level 3, east from the three-way intersection, then take north path
-Shinjuku Digital Shift, west of floating sphere
-Kowloon Level 4, after beating Flamedramon, open the right chest
-Asakusa Digital Shift, second area, go east on narrow path
-Valhalla Server, westmost part of the third area, on the outermost ring
-Nakano Underpass, southwest of the access point
-Ueno Digital Shift, left area's southwest corner
-Kowloon Level 5, second to last area, just south of the exit
-Tokyo Metropolitan Office, 30th floor's top right corner
-Tokyo Metropolitan Office, climb to the 39th floor from the 38th
That should be all of the Memory UP and Memory UP DX locations! In total you should have gotten 37 Memory UP items, adding the regular and DX variants together. If you did get them all, you will have 255 Party Memory. Then all the Digimon can be yours. Just be sure to use them responsibly.