Not all of the applications on Kasuga’s smartphone are unlocked in Yazkua: Like a Dragon. You need to unlock them by progressing through the story and starting side quests. The Sujidex is one of the applications that require you to interact with the second side quest in the game, and you activate it by visiting a particular location in ____. After you unlock it, your Sujidex becomes available to you, and you might recognize it from another pocket monster game.
To activate side quest 2 in Yakuza: Like a Dragon, you need to go to E. Central St. Proceed down the street, and you’ll encounter an NPC. Upon defeating them in combat, you’ll be approached by someone called Sensei. They’ll talk to you about a device they have and lead you to a new location to discuss working for him to use the device. You learn it’s called the Sujidex, and with it, you can collect data and information regarding the characters you randomly battle while wandering around Yakuza: Like a Dragon. It’s extremely familiar to battling Pokémon, where you learn about them after fighting them, but you don’t capture them.
After you agree to help them, you have to battle a red, green, and blue Sujimon individual, who are being paid to fight you to collect data. Following the end of this, the side quest called Who’s That Sujimon!?, you’ll unlock the Sujidex on your smartphone. Every battle you fight a new enemy type, it adds to your Sujidex. You can refer to the application on your phone to learn about what category they are, the enemy’s weaknesses, what they drop, and where you can find them. For players eager to farm certain items to craft, unlocking the Sujidex helps a lot.