Just a few things I've found playing with the Debug Flag editor.
Row/Column A0, 7-8. If turned on, it will have the Lava cave boss and 8th seal completed. Which is the cutoff you have to marry Eudenus to someone before he marries Pepe by default.
Row/Column 90, D. Will have the lava cave destroyed. So, if you didn't marry Eudenus to any of your characters, then he'll be married to Pepe.
During the game, after lava cave:
90, A = He says Farus' pals are his friends and that you can take his treasure upstairs. This happens with Nikita.
90, B = He tells whoever his bride is to shup up, and his bride threatens him and he apologizes. This happens with Yuma, Jenka, Melina
90, C = He says he's poor and everyone took off because he has no money. This happens with Iris.
90, D = He says what if he had married someone different, a strong woman soldier or a spiritualist. Pepe says she's happy to see you. This happens if you complete the lava cave without marrying one of your mercs to Eudenus.
During the ending:
90, A = He says he's elated, and suggests you get married to. If you marry him to Nikita.
90, B = Eudenus says he has a fine wife and the world is at peace. If you marry him to Yuma, Jenka, Melina,
90, C = He questions what marriage means to him. This happens if you marry him to Iris.
90, D = He has a fine wife and the world is at peace. Pepe thinks she's happy, but the world is at peace.
Any other character is either not available at the time (Joynus, Lovie, and Myra), or gives you the Ending B.
So I pretty much solved the mystery on my own
