Yes, she does matter! There are several builds, useful depending on your character build.

For example, my Elementalist, very powerful DPS-er, relies on Katarina to act like a meat-shield. Not to go in too much detail, she has Corpus skills (to make her having more HPs and Dodge chance, also everything damage-related - spells per hit, criticals - some of those are in Specter tree). From Sentinel tree, which gives bonus to my character, I took very little.

This Katarina is made for survivability and occasional kills (since main char is DPS-er). Not everything can be maxed, mind it, and some skills have (seemingly) diminishing return. Take whatever you need for one point, and add more, max even what it's most important. She is in melee mode with 'survive'.

Another approach is exactly opposite - putting her in Ghost mode and max Sentinel tree for bonuses which she giver to main character. This is more appropriate for melee chars, for example, who need attack speed and resistances more than Elementalist do.

Ranged Katarina is possible, too - I find her more fragile then previous versions, but some people play this way.

Also, do the attributes according to the way you play.

Hope this helps...