Oliver is such an unsympathetic character because of his attitude towards his brother Orlando. He is horrible towards Orlando for seemingly no reason, and even strikes him and throws his out of the house. Shakespeare most likely portrayed him this way for two reasons: one, to show social upheaval and the importance outside the forest, of birthright and society. Secondly, Shakespeare uses Oliver as a direct foil to Orlando.
Oliver is the so-called main villain in Shakespeares pastoral comedy As You Like It. Shakespeare shows him as an unsympathetic character primarily to glorify Orlando's role as the hero of the drama.
It also reveals the good traits of Orlando(with contrast to Oliver). It reveals the injustice done towards Orlando ; however at the same time, explicitly showing the evil - mind of Oliver.