Demetrius
Demetrius is the play's least sympathetic of the four young lovers at the outset — his cruelty to Helena in the forest is real before it is comic — but the enchantment transforms him into a figure of extravagant devotion that the play ultimately ratifies. His trajectory is the most structurally peculiar of the four: he ends the play under a fairy spell, and no one mentions it.
Appears In
Biography
Demetrius is the Athenian youth who holds Hermia's father's approval and none of Hermia's heart — a position he pursues with the full confidence of a young man accustomed to social sanction doing the work of personal appeal. Before the play begins he was himself in love with Helena, and abandoned her; this fact is established early and never quite forgotten, colouring his pursuit of Hermia with an undertone of inconstancy he is entirely unaware of.
In the forest, enchanted Demetrius becomes the most extravagantly devoted of Helena's sudden admirers — competing with enchanted Lysander for a woman he was recently ignoring, with a complete lack of self-awareness that is both comic and pointed. The play's joke about Demetrius is that his enchanted love for Helena is structurally indistinguishable from his natural love — it is all equally ardent, equally total, and equally contingent on something other than Helena herself.
The play's most quietly interesting point about him: Oberon's correction of the enchantment restores Demetrius to his original love for Helena, which means the magic that remains in him at the end is not the enchantment but the reversal of it. He ends the play loving Helena by fairy intervention, which the comedy treats as a happy ending and the audience is left to examine.
Motivation
Demetrius is motivated before the enchantment by social ambition and the approval of Egeus — he pursues Hermia because he is permitted to, and abandoned Helena because he chose to. Under enchantment he is motivated by the same intensity of feeling he previously directed at Hermia, now correctly aimed.
Personality
Demetrius is defined primarily by contrast: he is crueller to Helena in the forest than the comedy strictly requires, more extravagantly devoted to her under enchantment than is strictly plausible, and the beneficiary of a restoration that the play treats as entirely unproblematic. He is the lover whose arc depends most heavily on external intervention, which makes him both the comedy's object of mild satire and its most unexamined character.