in order to get the "Promise Card" which is required to marry. You have to get all of Haseo's party members in Volume 3. (yes, that means beating all of the Chaotic PKs in Areas to get Natsume, you MUST finish the game aswell, and lastly getting Tabby from the Forest of Pain) After getting all party members, CC Corp will email talking about Marriage. Then Aura will email giving you the promise card. THEN, (i suggest saving if you want to see all endings) give the card to someone you have MAX (big purple circle with 5 little ones) affection towards then their event will trigger the next time you log in to "The World" in Mac Anu.