Abigail is devious, unrepentant, trenchant and smart. In the first act, she is devious and smart in her manipulation of others. Her tactics are bullying and deceitful.
When Parris questions Abigail about why she was released from service by the Proctor, Abigail evades. When she is threatened by punishment for taking part in a nude midnight ritual in the woods with the other girls, Abigail not only swears the others to be loyal to her, she also initiates the lie that will ultimately animate the rest of the play.
Playing on the religious paranoia of her community (and the community's existing divisive feelings), Abigail spins a tale of witchcraft and convinces the other girls who had been dancing in the woods to go along with the tale.
"Abigail senses that the community of Salem, Massachusetts, is uneasy, that it suffers from societal tensions, and that it is prepared to believe that its internal divisions are the result of witchcraft" (eNotes).
In hatching this conspiracy, Abigail shows herself to be a powerful leader, although her methods are ugly. She is a bully and a tyrant, willing to use threats and also to bring harm to others if it means that she will get her way and avoid punishment for her own deeds.
Also, Abigail is unapologetic about having an affair with John Proctor. She is only a teenager when the two conduct their illicit romance, so much of the blame from a contemporary perspective falls squarely on Proctor and not on her in the context of their affair. (This would most likely have been the case at the time as well.) Even her willful insistence that the two rekindle their romance can be reasonably blamed on Proctor, who created expectations in Abigail that he could never honestly fulfill.
Notable characteristics in Abigail are nonetheless expressed in her conversation with Proctor in the first act. She believes that she can get her way, despite protest and despite the fraught morality of the situation that Proctor has now realized.
Proctor, gently pressing her from him, with great sympathy but firmly: Child—
Abigail, with a flash of anger: How do you call me child!
Proctor: Abby, I may think of you softly from time to time. But I will cut off my hand before I’ll ever reach for you again. Wipe it out of mind. We never touched, Abby.
Abigail: Aye, but we did.
Abigail is firm in her resolve (trenchant) and adamant about the rightness of her position/demands. After conniving to blame innocent others for acts witchcraft in order to escape punishment for things she actually did, Abigail shows herself to be morally numb in arguing that Proctor should continue to cheat on his wife by returning to his affair with Abigail.
In her eyes, it seems that anything she can get away with is morally acceptable. Essentially this means that Abigail is a character lacking a moral compass.