Orlick, is in the simplest terms, Joe's assistant at the forge. He then becomes gatekeeper for Miss Havisham until Joe has him fired from his post because Pip--correctly--believes him to evil and untrustworthy. In more complex terms Orlick is the contrast to Magwitch. Magwitch is branded a criminal, Orlick is not (at first). Magwitch has inner character that has good and decent qualities even if he did run into trouble with the justice system. Orlick has villainy and evil designs all the way through and has perpetrated harm against people, including a attempt at Pip's life, eve though he has never been taken up by the justice system.
Orlick's importance to the novel is two-fold in that he helps to emphasize and clarify (in a dark and murky novel) Dickens' theme relating to the sometime conflict between the twin goals of self-improvement and ambition. Self-improvement and ambition are both well and good as long as they don't lead to injustice and inhumanity in the areas of social standing, class consciousness and arrogant privileges of wealth. Self-improvement and ambition are good and useful as long as the incorporate loving kindness toward other human beings, humility and accurate assessment of self-worth, which are qualities even Magwitch demonstrated on his own level and which Orlick was devoid of having.
Orlick is a rather frightening character who helps Joe out at the forge; he is, like Pip, an apprentice in-training to become a blacksmith. At first, he just seems like kind-of a mean, scary guy that hangs around the forge quite a bit, and goes and gets slightly drunk at the local tavern every evening. But, as the story progresses, he ends up playing quite a significant role in some key events. First of all, Pip suspects him to be the one that irreversibly harms his sister. He comes home one day to find his sister hurt badly, and after that, she can barely speak or move. No one really knows what happens, but Orlick was suspiciously unaccounted for during the events, and his sister seems very animated whenever he comes around after that.
Later on in the story, Pip runs into Orlick again, and the nasty character threatens harm and murder. He traps Pip in an old shack in the middle of nowhere, and has full intentions of murdering him. So, overall, Orlick is a villian, one that Dickens uses to cause great harm and suspense in the story from beginning to end. I hope that those thoughts helped; good luck!