John was a Rogue and he had the ability to escape, hide and attack from the shadows. His Quiet Steps and Haze Veil allowed him to fight from the shadows and skills like Dagger Aura and Critical Hit allowed him to deal devastating damage with just his dagger.
From his borrowed skills, Muscle Strengthening and Strength Buff were the most straightforward. They provided him with raw strength. The difference between the two was that the Strength Buff provided him far more strength without a direct physical change to his muscles, but the downside was that it didn't last as long.
Berserker's Rage also worked like the other two, but on steroids and it should be used as a last resort, since his mind would always become clouded with violence and it was easy to lose sight of his real objective when using the skill.