for those who haven't read Kayn's backstory. here's the long short of it.
Kayn is a child soldier from Noxus, who joined the invasion of Ionia and while Kayn's entire squad was wiped out, he stayed alive and killed off the majority of the defenders he faced, even dropping his sword and picking up a significantly inferior weapon in direct combat, the sickle before the Noxan army joined him to sweep the remaining survivors.
Skip ahead two days later. Zed hears about the fight and finds a bloodied Kayn still there, left to die by the Noxian (even though Noxus won that battle but hey they probably just didn't like the kid very much).
Zed personally welcomed the kid into his League and started to train him, Kayn then learned to master EVERY WEAPON THE ORDER OF SHADOW HAD TO OFFER. This is a feat we Haven't even seen Zed preform simply because according to traditions you learn to master a single tool of your choosing (similar to Shaolin Munks who choose two). But Kayn learned all of them in the span of what we can only assume is a few years, why he even did it we don't really know because as far as we know Kayn heavily favors sickle like weapons but that doesn't really matter.
Anyhow, Kayn became a master assassin shortly after and even gained a reputation which made him arrogant towards others and hopeful that one day he could overthrow Zed as the head of the Order of Shadow.
Zed seemingly noticed this arrogance so he send Kayn out to basically a suicide mission, retrieving a Darkin weapon from the heart of Noxus so it could never be used against Ionia and he did that too.
- A Noxian child soldier who faught since he was 10 years old and the lone survivor of his first battle.
- A personal student, chosen by Zed himself within the Order of Shadow.
- A master of every weapon the Order of Shadow has to offer.
- Naturally gifted in learning the shadow Technique
- The owner of Darkin weapon which he stole out of Noxus
And to top things off he's incredibly arrogant.
Like seriously, does this not sound like timmy's first League of Legends OC?