What does 2 darkened hearts at the social tab mean?

[Maluno]

When you look at the social tab, there are 10 hearts heart outlines after each NPC. As you become liked by them you fill them from left to right.

On some NPCs though it shows 2 darkened hearts at the end of the line. It can happen with NPCs that i have no red hearts on the left, but also on ones that like me.

example: http://puu.sh/oGaAg/fe5b000c94.png

What do they mean?

[McGoodGreen]

Those are for the single people in town.

In order for you to raise their hearts above level 8, you have to give them a special item that becomes available at Pierres after you reach high enough affection with someone.

The item is a Bouquet and is cheap, 200g, once you gift it to your beloved, you will be going "Steady" and they will be your "boyfriend" or "girlfriend"

[Rio]

Those are eternal hearts. They won't disappear until they take damage. Its what keeps the singles alive before they pick up any red health.