Longevity is a Kindness deck specific effect card. When used, it will fully heal a monster of you choice (including an opponent's monster), give it Candy and gives it bonus HP equal to the amount of HP the monster restored.
This is especially useful for monsters with high HP and Taunt once they reach low HP, such as Gyftrot and Greater Dog. It can also be very useful on Mettaton. Due to his defence, it will be hard to take down. And if you give it Taunt via Glad Dummy and more ATK with Snowdrake to prevent Mettaton from dying by Muffet's Pet, he will be almost indestructible.
Longevity can also be good with Papyrus, as you can use it to fully restore him once he gets low on health from repeated attacks to kill of more of the enemy's board (if needed). If his HP gets high enough for this and survives the enemy's next turn, you can try using Madjick on him to give him a massive amount of ATK to attack the enemy directly with.
Before Beta 17.0, this card was a Rare. It swapped its Rarity with Soothing.