I think sassy or relaxed nature would be good .Both will minus speed which won't matter since you have prankster and if you think your foes are mostly special attackers use sassy but if physical attackers are more then use relaxed though I will prefer sassy as it has good defense but a weaker special defense .Its defense is 85 while it's special defense is 75 so the defense will be balanced. I think a balanced defense works better and anyways your whimsicott is almost completely defensive ( Which I believe). I am sure that this is the best advice Hope you like it and hope I have helped you