I can't believe this didn't hit me or anyone else sooner. Let me start by stating some rather obvious facts about shadow swords:
We know they absorb auras.
We know that after absorbing a certain amount, the original/dominant aura is lost the wielder becomes a combination of the first aspects he grasps from the cache within the sword.
Now for some less obvious facts:
Whenever a new dominant aura is established, it does not have to be important. Think of all the minor characters Rhue captured in his sword. Any one of these could have become Rhue's dominant aura.
So here's my theory about Dirk:
Whoever the original Dirk was, let's call him "Kird" for lack of creativity on my part, had a shadow sword. He lived for a longass time like most people with shadowswords do. Somewhere around Landorin, he killed some weak minded dipshit and absorbed his aura. This was the breaking point for his sword, and his dominant aura was replaced by the aura of the dipshit, or "Dirk." Dirk, now realizing he's standing behind a guy with a sword in his back, starts to freak out, and he's sitting there in the fetal position. The sword now realizes it can't use its previous wielder, due to his sudden change in demeanor and calls out for Jeruh... Dirk is the dipshit we are told by Jeruh is sitting there staring off into space. He's staring off into space because he just witnessed a guy die, and has no idea what the fuck happened.
Fifteen years or so later, age has finally caught up with Dirk. He still has the personality of Dipshit, but also some of the wisdom and knowledge of his former self. This is when he meets Rhue.
The end.