You can say my whole name. You can say Kat (most common). Or Kath. Or Kate. Or Katie (all rarities).
But never Kathy. That one will earn you immediate enmity for life.
I'm not a depressed comic strip character.
(this is a toot related to the multiple variations on my name coworkers have decided they like best, sometimes literally in the same conversation or email thread)
๐ฆ