I think the narrator of this excellent tale is alienated at every single turn. She is never allowed to fit in wherever she is, either thanks to Mrs. Van Hopper's snide put-downs and manner or the silence of Maxim, or the treatment from Mrs. Danvers and other servants. She is made to feel an intruder in Manderley, a character who could never possibly hope to live up to her predecessor, and is profoundly miserable and angst-ridden as a result.
The main character of Rebecca is not awarded a name because she walks in the shadow of Rebecca's ghost; this fact, alone, points to her position as an outsider, even in her marriage and new home. Before she meets DeWinters and because she has no family upon which to rely, she finds herself having to serve as a despised companion to a tyrannical mistress who treats her like a cipher. Because of the great difference in their ages, she struggles to overcome her naivete and innocence in order to earn her sophisticated husband's approval; her experiences at Mandalay seems to point to the fact that she is incompetently filling Rebecca's shoes when in fact DeWinters lives in fear that she will find out his secret: that he despised Rebecca and has murdered her. His secret insures the delay of an equal partnership and prolongs her alienation in the marriage. After she finds out the truth, she finds her place as his mate, but she and her husband are then alienated from society by their shared knowledge of his crime.
I'm going to treat Mrs. de Winter as the main character for the purpose of this discussion. (It's possible to interpret things differently.) Mrs. de Winter is alienated from her very name, as she's not called by it through a portion of the book. She is alienated from her family, because her parents died. In traveling with an American in France, she is part of a literally alienated party (not in home country). She's alienated from her position in the house through Mrs. Danvers' manipulation and through Maxim's failure to explain things (including himself) fully.
The main character, as in the second Mrs de Winter struggles with alienation because she doesn't have any roots. She has no family or friends. She consistently experiences alienation as she comes into contact with Maxim and his class of aristocratic associates - who are much different to her own social status - and she experiences self-consciousness because of her lower class status and her inexperience of the ways and manners of the higher class and procedures of Manderley and making telephone calls, writing letters etc.