Katarina needs utility/defense

Kat has legit no defense yet she's an assassin that stands still for 2.5 seconds ulting someone. She has no built in sustain (only Gunblade) and it isn't that huge until you get damage. I don't get why kat is literally in the middle of 5 enemy champions yet she can't even get any shields or heals or "unstoppable" it literally makes no sense to keep a melee champion which is also an assassin no defense.