Kayn using mana doesn't make sense

It doesn't quite make sense to me why is a mana spending champion. Don't get me wrong, I have nothing against him being limited by a form of resource, but I just don't understand why it had to be mana.

If we consider the fact that is the student of , aka a student of shadow ninjutsu, then you'd think that Kayn would be taught the same techniques as Zed himself. Zed uses energy to use his shadow abilities, which is resource that focuses on physical limitations and is also used by all the other ninjas ( (?)). If we take a look at Akali she also uses energy as a resource, and also focuses on shadow ninjutsu as her title is 'The Fist of Shadow' after all.

So why does Kayn not use energy as a resource? Why does Kayn have to use mana, a form of energy that manifests on the users mind, to cast his shadow abilities? If Kayn decides to go shadow assassin, he gains the ability to spawn a living shadow just like Zed, but this one uses mana. Not to mention that Kayn/Rhaast use no form of magic when they use Q and W. Another thing I really feel like was a missed opportunity, is that Rhaast also uses mana and not a similar system like does, or at the very least rage.