Might be a stupid question but I actually haven't figured this out yet.
#How do I unlock the final 2 hearts with an npc?
zenith kraken
glossy condor
1st you need to have viewed all of their heart events. After you've done that, you need to buy a locket from the blacksmith. This will "propose" a relationship with them. After you do this, you need to give them gifts, talk to them etc to get their final two hearts. You then need to view their heart events that are available. After all of that you can finally propose marriage to them, by buying the ring from the blacksmith.
zenith kraken
I've watched all the heart events so thank you! I'll do that now