I know this has been asked already, but I've been trying for hours now and for the love of god, I can't figure out the Reunion ending. Isn't there any hint you could give?
If you're still stuck on it, the Reunion ending is attainable after last having saw specific endings with Rina & Ace -- a hint for the endings required is that they are one of the more positive ones. As the bartender, you should also be making choices that urge Dawn towards reconciliation and change.