In order to get locked into Zen’s route in Mystic Messenger, first you have to make sure that you’re playing the Casual Story mode. This is the first mode you can unlock upon starting up the game. For the first four days, you’ll have to collect as many Hearts as possible from Zen to make sure that you get his route by the time the route split rolls around.
With Zen, dialogue options that boost his ego will help because he’s generally a pretty narcissistic guy. For the first few days, you’re free to flirt back with him whenever he does so, and you should definitely always take his side whenever he’s arguing with Jumin in the chatrooms. It’s also a good idea to be supportive of him, and reassure him that he’s chosen the right career path.
After you’ve unlocked his route, things get a little bit trickier. You’ll still want to be supportive, but you have to be careful not to come off as an overenthusiastic fan who only likes him because of his fame. Zen is looking for a reliable friend, and you have to fill that role for him. You should also try to convince him to behave in a more genuine manner, and also patch things up with his family. Jaehee will occasionally step in and express her concerns about your relationship with Zen, but you should tell her that you understand her point of view without being dismissive of her. Lastly, remember to bite his hand when the opportunity arises to unlock his good ending.
These general rules should help you out with unlocking his route and good ending in Mystic Messenger. If you have any other tips, feel free to sound off in the comments down below.