Henrietta Stackpole

Henrietta Stackpole is an American journalist and friend of Isabel Archer, who comes to England to visit Isabel and to collect stories about foreigners.  In the preface to the 1908 New York edition of the work, James essentially apologizes for the character of Miss Stackpole:

As to Henrietta, my apology for whom I just left incomplete, she exemplifies, I fear, in her superabundance, not an element of my plan, but only an excess of my zeal. So early was to begin my tendency to OVERTREAT, rather than undertreat (when there was choice or danger) my subject. (Many members of my craft, I gather, are far from agreeing with me, but I have always held overtreating the minor disservice.) "Treating" that of "The Portrait" amounted to never forgetting, by any lapse, that the thing was under a special obligation to be amusing. There was the danger of the noted "thinness"—which was to be averted, tooth and nail, by cultivation of the lively. That is at least how I see it to-day. Henrietta must have been at that time a part of my wonderful notion of the lively.

James calls Miss Stackpole "an excess of my zeal," her role being to add humor, which she absolutely does.

Isabel calls her "decidedly pretty" while James tells us she has a clear but loud voice (the loud found offensive by Mrs. Touchett).  She is strongly American and judges everyhting by American standards, causing the English characters in the novel to think her rude.  Though "thoroughly good-natured" (Ch. 10), she looks people directly in the eyes, threreby making them uncomfortable;  she doesn't knock upon entry; she is told by Isabel that she has "no sense of privacy" (Ch. 10); and she is not timid in voicing her opinions, particularly when she disagrees with someone, such as when she tells Warburton, "I don't approve of a privileged class" (Ch. 14).  Due to her "aggressive frankness" (Ch. 11), no one but Isabel likes her so far in the novel.

One role Henrietta plays in the novel is a comical one.  Her exchanges with Ralph are hilaious, particularly because while she accuses Ralph of not being serious enough, she herself is too serious.  In one of the funniest scenes, in which Henrietta insultingly calls Ralph "European" (Ch. 10), despite his American birth, Henrietta tells Ralph he needs to marry because it is his duty, advice Ralph mistakens for a proposal which he foolishly accepts.  Henrietta, in turn, is offended and Ralph embarassed.

Another role Henrietta plays early in the novel is to inform the reader that Isabel has changed since her arrival in England (though Goodwood intimates that the change occurred when she found out she was to go to England). 

"Yes, you're changed; you've got new ideas over here," her friend continued.

"I hope so," said Isabel; "one should get as many new ideas as possible."
"Yes; but they shouldn't interfere with the old ones when the old ones have been the right ones."
Isabel turned about again. "If you mean that I had any idea with regard to Mr. Goodwood—!" But she faltered before her friend's implacable glitter.
"My dear child, you certainly encouraged him."
Isabel made for the moment as if to deny this charge; instead of which, however, she presently answered: "It's very true. I did encourage him." (Ch. 11)

Exactly what these "new ideas" are is not clear yet, though it seems to have something to do with Isabel's ideas about marriage.  Apparently, Isabel was willing to give herself to Caspar Goodwood, an American friend, but has since changed her mind about a possible marriage.  Isabel and Henrietta seem to differ on one's obligation to marriage.  While Henrietta sees marriage as a duty, Isabel does not see marriage as an obligation to everyone.

"I am not sure I wish to marry anyone" (Ch. 12).

Her reason for possibly not marrying may be her fear of relinquishing her independence.

"The idea of diminished liberty was particularly disagreeable to Isabel at present" (Ch. 13).

Isabel fears there is life to live which she would miss out on by marrying at this moment.  For Henrietta, Isabel is neglecting her duty.

The above painting is The Symphony in White (1862) by James Whistler.