Tutorial Devil Survivor 2: Record Breaker - Obtaining SpotPass demons by hex editing

It's pretty simple.
If you have a 3DS, you'll need Custom Firmware (CFW) to extract your save file (using either Checkpoint or JKSM), which should come as 5 files. The one we need is save0.dat. After transferring the file to your PC using an SD card reader or FTP, you'll open it with your hex editor of choice. I'm on Linux and I'm using GHex, but any hex editor will work for this.
Search for the hex codes below using CTRL + F (or the search function button), and replace the first three '00' values with 'FF'. This will enable the demon in your Compendium. However, we're not done yet, as we need to set at least Level, Speed and Moves for the demon, otherwise it won't show up.
If you just want the demon to appear in your Compendium as a Lv. 1 demon with no EXP, set it 'FF FF XX XX FF 01 32 04', where XX XX is the demon's ID, and nothing else. But if you want to configure the stats, EXP, and other things, you need to edit them manually:

FF FF 26 01 FF 45 2D 03 00 00 17 1A 11 15 5A 02
1D 01 09 00 19 00 20 2D 30 00 00 00 00 00 00 00

This is the hex code for a Lucifer Frost, edited according to a 103% save file I found around. Let's take a look at it:

Red refers to the enabling values for your demon. Any demons you want to enable NEED to have all these three values set as 'FF'.
Green refers to your demon's level. You'll need to have a hexadecimal conversion table on hand, since the actual values in the game will be converted to decimal values. So, if we have '45', in-game we will have Lv. 69.
Orange refers to your demon's Speed, which is an invisible value in-game that determines whether your demon gets an extra turn or not. By default, that value is '2D'.
Yellow refers to your demon's Moves, which is the amount of tiles it can move in combats. Most demons have this value set to '03' or '04'.
Blue is your demon's amount of EXP. I'm not sure how it is calculated, so you'll have to test values and see what matches your needs best.
Purple refers to your demon's stats, which are ST, MA, VI and AG respectively. These values are converted from hex to octal, so you'll need to have a conversion table on hand for this too. If we have '17' for example, in-game we'll have 27 ST.
Dark Orange refers to your demon's HP. This one is also a bit of trial and error, since the game calculates HP based on the demon's base HP and Vi. The formula for this one is Displayed HP = Base HP + (Vitality (Octal) × Factor), if you'd like to actually calculate.
Light Blue refers to your demon's MP. The hexadecimal value you put is converted to decimal (8 bit integer), then to a 16 bit integer that is displayed in-game, so you may want to look up a website to do that for you.
Brown refers to your demon's Commands. There are a total of three Commands you can choose, and each value corresponds to a specific move. Thankfully, I have all the Command Skills codes thanks to a cheat file in GitHub, and will provide them below:
Dark Purple refers to your demon's Passive skills. There are a total of three Passives you can choose, and each value corresponds to a specific move. Thankfully, I have all the Passive Skills codes thanks to a cheat file in GitHub, and will provide them below:
Aquamarine refers to your demon's Auto skill. You can only pick one Auto skill, with the value corresponding to a specific skill. Thankfully, I have all the Auto Skill codes thanks to a cheat file in GitHub, and will provide them below:
Black are filler bytes and they don't need to be adjusted.

Fusion Pack 1:
Fallen Dantalian -> 00 00 35 01 00
Touki Momunofu -> 00 00 25 01 00
Megami Ishtar -> 00 00 0E 01 00
Fairy Troll -> 00 00 19 01 00
Fairy Spriggan -> 00 00 18 01 00
Avian Kau -> 00 00 14 01 00

Fusion Pack 2:
Tyrant Lucifer Frost -> 00 00 26 01 00
Divine Dominion -> 00 00 33 01 00
Jaki Dokkaebi -> 00 00 1C 01 00
Genma Roitschaggata -> 00 00 17 01 00
Wilder Catoblepas -> 00 00 16 01 00
Kishin Okuninushi -> 00 00 1B 01 00

Fusion Pack 3:
Vile Seth -> 00 00 36 01 00
Drake Fafnir -> 00 00 11 01 00
Tyrant Lucifuge -> 00 00 1A 01 00
Megami Black Maria -> 00 00 0F 01 00
Touki Ongyo-Ki -> 00 00 1D 01 00
Beast Cabracan -> 00 00 15 01 00

Fusion Pack 4:
Fiend Mother Harlot -> 00 00 34 01 00
Avian Rukh -> 00 00 13 01 00
Femme Jahi -> 00 00 1F 01 00
Haunt Macabre -> 00 00 1E 01 00
Divine Sraosha -> 00 00 12 01 00
Snake Yurlungur -> 00 00 10 01 00