Not much panels to tell the whole story, but this is basically how it went (Kai and Duran were 20 and 21, respectively):

Kai was the appointed guard to a man caught trespassing the castle. He was standing outside while the prisoner

was questioned, and heard everything: His name was Xen, he was hired to kill someone, and he wasn't working

alone (basically, he gave a lot of info, hoping they would release him).

Xen was able to reveal that his partner was Duran, gave a description of him - and even his age - before the interrogation

suddenly stopped. Kai went inside to see what happened... And saw someone standing over Xen. It was Duran - he had

slit both Xen's and the officer's throat.

And that's how they first met! XD I didn't want to make this long, but obviously Duran wasn't able to silence Kai as well,

and the assassination attempt had to be aborted because of Xen's confession. So, in the end, Duran had to run.