Evil stepfather, scheming sidekick, sniveling coward, general nuisance. Cornelius is the thorn in the side of anyone and everyone who gives a hoot about Jim, or, you know, basic human decency. He is the manager of Stein's Patusan trading post, whom Jim is sent to replace. This failure makes him bitter and weak, and Marlow wastes no time in describing him as straight-up bad:
Cornelius was creeping across in full view with an inexpressible effect of stealthiness, of dark and secret slinking. He reminded one of everything that is unsavory. His slow laborious walk resembled the creeping of a repulsive beetle [...] (29.2)
So we know he's creeping, stealthy, dark, unsavory, and repulsive. He wants to regain some power in Patusan, and he tries to use bullying to do it. He takes advantage of those weaker than he, most notably his wife and stepdaughter. We never learn exactly how Jewel's mother ended up marrying Cornelius, but we do know that it was a huge mistake. Cornelius turned out to be an abusive jerk, who constantly harassed his wife, even on her deathbed:
She went on explaining that, during the last moments, being alone with her mother, she had to leave the side of the couch to go and set her back against the door, in order to keep Cornelius out. He desired to get in, and kept on drumming with both fists, only desisting now and again to shout huskily, "Let me in! Let me in! Let me in!" (33.6)
He treats Jewel much in the same way:
It appears Cornelius led her an awful life, stopping only short of actual ill-usage, for which he had not the pluck, I suppose. He insisted upon her calling him father – "and with respect too – with respect" he would scream, shaking a little yellow fist in her face. (30.1)
Lovely. In a book with lots of fathers and father figures, Cornelius is one bad daddy. Just compare him to Jim's own father, Marlow, or Doramin, all three of whom exhibit all the traits you'd want in a papa.
As if that weren't enough, Cornelius secures his position as the king of the jerks by betraying Jim (and most of Patusan, for that matter) and allying himself with Gentleman Brown. This one traitorous decision leads to the deaths of Dain Waris and Jim, not to mention Cornelius himself.
In Chapter 21, we're told that Cornelius is "Malacca Portuguese." Malacca was a Portuguese colony in Malaysia, which presents the possibility that he might be half-white, half-Malaysian. In any case, he is definitely not English.
In the passage we mentioned above, in which he antagonizes his stepdaughter, Marlow mentions his "yellow fist." That his non-English, non-white heritage is mentioned in the same breath as his "unsavory" personality traits is telling, because of the racially charged atmosphere of the time. Might Conrad be playing up Cornelius's ethnic differences as a parallel to his general bad behavior? Many critics, like Chinua Achebe, have had a beef with Conrad over his portrayals of race in European colonies. But is Conrad making a connection between Cornelius's ethnicity and Cornelius's behavior, or is he merely reflecting the way people might have thought at the time?