Thanks, I already gave the heart a makeover (inner white boarder to fit to the text) and added the full name of the character to the top of the information page.
The heart is kind of taken from harvest moon/pokemonami, you can increase base happiness of all characters by interacting with them in your base, agreeing with their point in cutscenes and giving them food/items they like.
As of now, if your party members are happy, then they gain quicker healing from some conditions in battle as well as higher chance of critical hits.