Before you can unlock Jumin’s good ending in Mystic Messenger, first you have to unlock his route in the game. In order to this, you have to gain the most number of Hearts with him. I advise focusing on getting his Hearts during the first four days of the game, and ignoring the rest of the characters to increase your chances of getting locked into his route. Also, do note that his route is only available in the Deep Story mode.
To get Hearts for Jumin, you have to be really positive about cats in general, and his own pet cat, Elizabeth III. You’ll also want to take his side in arguments, and be understanding about his point of view. Jumin also likes confident people, so you should pick the dialogue options that make you seem smart, but also humble at the same time.
Once you’ve unlocked his route, things start getting a bit more serious. Whenever he’s going through tough times, you have to show your emotional support for him, and do not be mean or unreasonable. When he starts getting overbearing and tries to keep you at home, it’s fine to tell him straight that you have to get ready to organize the upcoming party, but you can’t be mean about it, and you can’t scold him. The key here is to be understanding. You should also not gossip to the other characters about his possessive behavior.
Following these general rules should allow you to get the good ending for Jumin in Mystic Messenger. If you have any other suggestions that would help, feel free to sound off in the comments down below.