Happy Heart is a badge that slowly and automatically restores HP to Mario during battle.
In Paper Mario, it can be traded from Merlow for 8 Star Pieces. It can also be found in Jade Jungle. It uses 3 BP.
In Paper Mario: The Thousand-Year Door, it can be found in Petal Meadows and traded from Dazzle for 4 Star Pieces. It can also be stolen or dropped from Koopa Troopas. It uses 2 BP.
In Paper Mario