If you are indeed writing an essay, your job was likely to choose an outsider and prove it. I imagine your teacher would take any character you choose as long as you can prove it.
So, you could choose Abby. I know she is a ring-leader and seems on the inside, but really these circumstances run her completely out of town. She had no parents, she was sort of the bad kid who had to live with her preacher relative, and she caused a marriage to spoil. You can't be more on the outside of Puritan society than Abby.
You could choose the Putnams. This family was the whole reason for why the accusations got started. They put their daughter Ruth up to accusing people of witchcraft. Why would people do this? They weren't getting their own way. The rest of the church did not agree with the choice of their brother-in-law to be the new preacher, so they sabotaged everyone else.
If I was you, I would write about Mary Warren. She was the Proctors' servant after Abby was kicked out. Mary was a typical teenager who wanted to be liked. She wanted to go along with the girls, but also wanted to obey her employers. She got put into a position in Acts 2 and 3 that made her an outsider to everyone that mattered to her. No one could completely trust her anymore. She turned on her friends to help her employers and turned on her employers to help her friends. I think Miller positions Mary as the outsider to make readers and viewers realize the tough spots we can put people into.
Rev. Parris could also be an outsider. Miller portrays him as new to town in the beginning. He is terribly concerned about his reputation and we see this in the opening scene. The people don't necessarily respect him. Proctor vehemently opposed how Parris preached. He made a comment in Act 3 about how Parris doesn't even mention God in church anymore. To make matters worse, Parris isn't really accepted by the magistrates and Rev. Hale. His word means nothing.
In my opinion, after reading the crucible and writing an essay based upon it, I would actually think of Abigail as the outsider... Even though she is a main central character, she is an outsider in many ways:
Her parents were killed in front of her and she was sent to the village at a young age; she is more provocative than all the other young girls, making her seem more mature, and simultaneously seem to be, for lack of a better phrase, on the outside looking in.
Moreover, her desire to ruin Elizabeth Proctor's good name shows an evil, spiteful, vengeful nature that she also uses to lure the other girls into pretending they can 'see' the devil in certain villagers (use part of her speech in the first part where she threatens them in the church about how she watched her parents die, "sharp knife"). This is not only significant of the fact that she differs in the kind nature of others in the village, but it also puts her in a position where she is on the outskirts of the devout Puritan society.
In conclusion, you can use all these reasons and develop them for your essay to show that just because she is a central character does not necessarily mean she is not an outsider, on the contrary, she could not be more of an outsider, and the previous reasons, among several other reasons, can verify this and give evidence to this.