What is a demon's "personality" like?
Demons have personalities just as any human being has a personality. They are, after all, persons, except that they do not have bodies. We can know demonic personalities, and we can engage them in a battle precisely because they are personal beings, not impersonal forces of evil. This always becomes clear in situations where the demon uses the voice of the possessed individual to speak to the exorcist. The voice is the same as that of human victim, but the personality behand the voice is different.
Each demon's personality is related somehow, mysteriously, to his function. A demon of anger will speak, well, angrily. Demons of pride will insult and mockingly laugh at the exorcist and blaspheme the things of God. I once heard a demon of bigotry shout out obscene remarks about minorities and other races; a demon of retaliation once yelled at me vainly, "You'll pay for this." A demon of profanity used strings of curse words; and most curious of all, a demon of greed once asked me. "What's your price" (i.e., to stop the exorcism)?
It is important to discern the personalities of the stronger demons carefully in an exorcism. Their particular concerns, their relationship with the victim, their phobias and hatreds will give the priest a good sense of what their "function" is and how then to attack them most effectively. The Rite of Exorcism traditionally advocates finding the thing that bothers the demon most and repeating it. However, an exorcist must not manifest interest in the demon's personality as such or indulge in questioning the demon out of curiosity. In fact, the Rite of Exorcisms forbids it. The priest's only interest in a demonic personality is that he may more deeply understand the mystery of the possession in order to liberate the victim from the demon's power.
Fr. Thomas J. Euteneuer "Exorcism and the Church Militant" page 30, question 14