Arxus Rogue is a very powerful class in Rakenzarn. It is also Kyuu's primary class whenever he's in the storybook's world.
The class has an uncanny ability to wield all kinds of weapons and execute a variety of attacks. In addition, the class can use both physical attacks and magic spells. As Pete stated in the Untold Prologue, the class is very rare since it's very hard to achieve it. One must be very skilled and strong in terms of combat and mentality.
Unfortunately, Kyuu is truly inexperienced with such things so he's unable to utilized the class' potential. As a result, it renders him the weakest among the party. Kanata said that Kyuu needs to train himself in order to achieve the class' potential.
In Version 1, Kyuu needed to digest magic seeds (which increased his different attributes), learning moves from various characters, finding Skill Scrolls (magic items that allow anyone who reads it to learn a skill in an instant) and collect many weapons.
The catch though, is that some skills could only be used when a certain weapon type is equipped. For example, a skill called 'Double Slash' could only be used when a sword-type weapon is equipped. This forced the players to be very careful what weapons and skills they wanted to use in combat.
Versions 2 and 3Edit
In Version 2, players are able to use both weapons and skills freely. Each weapon that Kyuu can use has different (or even the same) skills and attacks via the Soul Points system.
In addition, players can seek various trainers or mentors to train Kyuu's stats and even grant them new weapons and skills to use. Training under mentors requires different quests to initiate the next session. By completing all the trainer's exercises, they grant Kyuu his very own Soul Break. Piccolo, for example, will give Kyuu the Namek Soul, granting him the Soul Break 'Oni Hado.'
Version 3 expands upon this by granting Kyuu the ability to use any weapon or armor in the game except those that are exclusive to a single character, granting the player the ability to further customize Kyuu to their own playstyle.