You have asked a very interesting question, because when analysing the characters of the Athenian lovers in this play (of which Demetrius is one), it is very hard to work out what their true nature is as it is changed so often thanks to the somewhat mischievous acts of Puck in interpreting...
You have asked a very interesting question, because when analysing the characters of the Athenian lovers in this play (of which Demetrius is one), it is very hard to work out what their true nature is as it is changed so often thanks to the somewhat mischievous acts of Puck in interpreting the orders of his Lord, Oberon, King of the Fairies. Thus to really think about the character of Demetrius we need to examine how it is revealed before the lovers fall in and out of love with each other.
The best place to start therefore is Act II scene 1, when Demetrius enters the stage followed closely behind by Helena. Notice how Demetrius speaks and addresses Helena:
I love thee not, therefore pursue me not...
Hence get thee gone, and follow me no more.
Demetrius is therefore very harsh and blunt with Helena - we can imagine how he would be acting out these lines and his expressions of disgust and revulsion when speaking to Helena whilst looking for Hermia. Notice how Demetrius' language only worsens when talking to Hermia in response to her faithful and constant love:
Tempt not too much the hatred of my spirit
For I am sick when I do look on thee.
We can see that Demetrius is almost violent in his hatred of Helena, which reveals him to be a very unreasonable and ungentleman-like individual. Let us not forget that it was he who, in Lysander's words,
Made love to Nedar's daughter, Helena,
And won her soul: and she (sweet Lady) dotes,
Devoutly dotes, dotes in idolatry,
Upon this spotted and inconstant man.
Therefore the character of Lysander seems to be established as a ruthless, inconstant and fickle lover who, having won the heart of Helena, now cruely spurns her in favour of his latest conquest, Hermia. Of course, during the course of the play Helena gets her revenge in a way, as Demetrius and Lysander are compelled to love her thanks to Puck's magic and Hermia is left bereft. This all goes to underline and expose the shallowness of the emotion called love, and how fickle it actually can be.