What is Kassadin's identity?

Your job is to be the antithesis to THE PROPHET OF THE VOID! (Peace be upon him.)

Resident of the Garbage Tier.

That's why you build Rod of Ages, so you don't blow up as easily. Zhonyas is pretty core, even against AP mids personally, because you get in, unload your rotation, zhonyas, and then either rotate again or ult out.

your job as kass is to jump into the back line (once you do enough damage) and kill it all its pretty easy once you get roa and ludens

Anti-Mage. Jump in mage's faces and eat their souls.

As Kassadin, you are forever doomed to hope for an extremely strong lategame that will never come. But the 1/100 games where you do hit late, you will carry extremely hard. Also, build on him for endless mana in teamfights.