For a lot of the play, creepy Krogstad seems to be a grade A villain (A, of course, is for antagonist). He is the force that our protagonist, Nora, is constantly butting up against. He is the foe to be defeated. Krogstad brings the trouble into Nora's seemingly perfect world by threatening, blackmailing, and turning Nora's life into an all around hellhole. It's pretty ironic that all of his shenanigans are motivated by a desire to be seen as a respectable person by the community.
In the end, however, Krogstad is reformed. When his old flame, Christine, offers her love, Krogstad is reborn and retracts his threatening letter to Torvald. It's too late for the Helmers, though; the damage has been done. We called him a villain earlier, but, now that we think about it, maybe he's a hero. If he'd never put Nora through all that turmoil, she'd never have woken up to the emptiness of her life. We still wouldn't invite him over for a barbecue, but there's no denying that, without his plots and scheming, Nora would still be nothing but a doll.