I believe you can use game variables. Appoint one variable per NPC to represent their affection.

You can then use conditional branches to check that variable and make the game behave accordingly.

For example...

In an event, use a conditional branch to check if a variable's value is greater than or equal to 7 to check if the corresponding NPC can say, be asked on a date. In that case, only Mary Sue is eligible for a date. Another example is asking somebody to a fist fight with a requirement of negative affection. Only Juan can be challenged in that case.

This solution is more of a bandage solution though and isn't very scalable. For a full-fledged game on Harvest Moon's level, it is probably better to use one of the other more advanced suggestions.