Demetrius, like Lysander, is a young gentleman of Athens. Unlike Lysander, Demetrius is shown to be fickle with his romantic attentions even before the influence of fairy hijinks. Although Demetrius is promised to marry Hermia by Egeus’s decree, a rumor circulates Athens that Demetrius had a previous relationship with Helena. This story is widespread enough that the Duke believes it, and Helena later confirms that, at the very least, Demetrius let her think he was interested in her. This backstory possibly paints Demetrius as more of a “bad boy” than Lysander, but it mostly establishes and explains Helena’s lovelorn longing for him at the beginning of the play.
Many of Shakespeare’s other plays feature characters whose specific personalities and motivations drive their unique plots. A Midsummer Night’s Dream does not need its four lovers to have any more unique motivations than to be in love with each other. Demetrius is the only character to remain under a spell by story’s end, but the plot proves this to be the best thing for everyone to get their happy endings, as is typical for Shakespearean comedies, which almost always end with weddings. Rather than emphasizing anything sinister in Oberon’s meddling, the play treats Demetrius as misguided in his love for Hermia and contented after he is made to fall for Helena. If the play were not a comedy, it might sit uncomfortably that Demetrius should be magically forced into loving Helena. As it is, Demetrius, Helena, Lysander, and Hermia are all simply agents of the plot, so the audience can be satisfied when the lovers end up neatly paired.