Helena & demetrius
In the beginning of A Midsummer Night's Dream, Demetrius plans to marry Hermia, Helena's best friend. However, Hermia does not return his love. Helena, on the other hand, is extremely in love with Demetrius and is jealous of Hermia for being able to gain Demetrius' affection. Helena's feelings for Demetrius are so great that she would allow him to mistreat her as long as she could be near him. In Act 2 Scene 1 she says,
"I am your spaniel; and, Demetrius,
The more you beat me, I will fawn on you:
Use me but as your spaniel, spurn me, strike me,
Neglect me, lose me; only give me leave,
Unworthy as I am, to follow you.
What worser place can I beg in your love,—
And yet a place of high respect with me,—
Than to be used as you use your dog?"
At the end of the play, Helena is successful in gaining Demetrius' love as a potion has been administered to him that made him attracted to her.
Helena
Even before the play, Helena displays great love and affection toward Demetrius; however, Demetrius fell in love with Hermia and left Helena in despair. Helena demeans herself in chasing after Demetrius even when he scorns her, but this demonstrates her constant love for him. Demetrius's abandonment of her caused Helena to lack self-confidence and self-respect, going so far as to tell Demetrius that she'd love and follow him even if he treated her like his dog. She continued to be insecure about his feelings for her toward the end of the play; when he does say he loves her, she assumes he is mocking her. After all, he had never loved her before, so it seemed absurd to think those feelings had changed. Helena's personality throughout the play is described as loyal and desperate, and as one who loses themselves in love. She is tall and blonde, which sets her physically apart from Hermia. Her undying love for Demetrius is her defining trait throughout the play, and her desperation adds to this quality.
demetrius
Before the play begins, Demetrius is in love with Hermia, and gains her father's approval for marriage. Hermia does not return that love though, which parallels Helena's unrequited love for Demetrius. He can be cruel sometimes, an example being when he threatened to leave Helena in the forest. There is also no sign that he would have ever come to love Helena without the influence of the love potion. Demetrius throughout the play is crude and demeaning, which is a large part of his personality and character. Demetrius often disregarded Helena's feelings toward him, or in some cases, going so far as to verbally insult her and ask her to leave him. These parts of his personality do not make up his entire character, however. Once the love potion is given to Demetrius, he becomes entirely devoted to Helena and drowns her in compliments and adoration.