Longevity is a Kindness deck specific effect card. When used, it will fully heal a monster of you choice (including an opponent's monster), then it gives an 75% bonus HP to it. It was basically the opposite of Bravery's Aaron's Secret until Beta 7.5.
This is especially useful for monsters with high HP and Taunt, 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, in multiple ways. You can use it to keep Papyrus healed up so he can keep on attacking, but you can also make use of his already high HP by increasing it a lot more and then use Madjick to give him an enourmous ammount of ATK, able to kill your opponent.
Before Beta 17.0, this card was a Rare. It swapped its Rarity with Soothing.