The worst thing any Unovan can say is "May you become a Yamask when you die." These ghost pokemon endure something far less than an afterlife, for although ghosts by definition have something which keeps them too tied to this world to move on to the next, most of them at least learn to enjoy their time in their new forms. For Yamask, however, happiness stops at death; all that is left for these pokemon is regret and mourning. A typically despondent Yamask will cry when they spot their mask for it drives home their present sorrows. In those rare instances where a Yamask distracts itself with other things, the sight of their own face in life twisted into a mask will still remind them of all that they failed to do when alive.
There are two known ways to a cheer a Yamask up. The first is to embrace it, for the warm touch of the living reminds these pokemon that although they are dead, they still can play a part in this world, and therefore need not mourn for the past. This method is difficult, however, as Yamask are ghosts, and the arms of most living creatures (including humans) simply go through them; only a few scrappy pokemon such as Kangaskhan can offer this consolation.
The other way to make Yamask happy is to turn their afterlife into an adventure. Although a lone Yamask does little but float in place and cry, Yamask are still pokemon, and are capable of all the battles and journeys this implies. A Yamask with friends and a trainer will slowly forget the pain of their past life, a process which is completed when they evolve into Cofagrigus – for Cofagrigus, although sometimes menacing, are as playful and happy ghosts as the Gengar.