Kassadin is a melee mage, the only kind of mage champion who uses melee, to be precise. You may think this is cool, its not. It just makes farming at the start that much more dangerous and tricky, which i believe is why he falls under "HARD" on difficulty.
However, kassadin one of the best mid and late game champs to date, if you can come out of the laning phase pretty well farmed or even fed, you can get some serious kills and make a huge difference in team fights. He is a lil tricky cause hes squishy but overall hes an amazing champion defined by his 4 sec CD ultimate which combines damage and movement. Ill go more into that later. There are two really good guides on here already, but i build my kass a lil different and i wanted my own build on here so if you happen to be reading this, i highly reccomend you read through the two highest rated kass guides on here, both of which were helpful when i started playing this beast of a champion.