Slender and simple, frequently soundless, she was somehow always in the line of the eye—she counted singularly for its pleasure. (220.127.116.11)
Kate Croy is an attractive woman. The book is clear about that on many occasions. But another aspect of her appearance is the way she strategically is always able to get into your line of sight. The fact that Kate takes a special pleasure in this suggests that she very much enjoys to be looked at. She knows she's a hottie.
He judged meanwhile her own appearance, as she knew she could always trust him to do; recognizing, estimating, sometimes disapproving, what she wore, showing her the interest he continued to take in her. (18.104.22.168)
Kate usually likes to be looked at, but not when it's by her father. The man is constantly judging her. At least this makes Kate feel like the old man still has some sort of interest in her, though. Her sister Marian, on the other hand, gets no attention at all from their father.
It gave him pleasure that she was handsome, that she was, in her way, a sensible value. (22.214.171.124)
Kate's father takes pleasure in the fact that she's pretty. In his terms, this makes her a "sensible value," or someone that might be able to marry a rich man. Of course Lionel Croy doesn't care about the benefit Kate might get from marrying into wealth—he's just interested in how much money she can send home to her dear old Dad.
She saw, and she blushed to see, that if, in contrast with some of its old aspects, life now affected her as a dress successfully 'done up,' this was a matter of ribbons and silk trimmings and lace, was a matter of ribbons and silk and velvet. (126.96.36.199)
Kate's miserable upbringing has caused her to take a fairly cynical approach to the world. She's been taught to think of herself as just a pretty face. As a result of this she thinks of value in terms of surface appearance. A life of luxury has affected her in the way that a really pretty dress would: it's made her more attractive. In Kate's skewed perspective of the world, this means that wealth has made her more intrinsically valuable.
It was a little the fault of his aspect, his personal marks, which made it almost impossible to name his profession." (188.8.131.52)
Merton Densher is a handsome guy. But in a town where appearances mean everything, he is strangely difficult to pin down. You'd be able to tell a banker by his suit and watch, or a Catholic priest by his collar. But Merton has a way of dressing and acting that makes his profession tough to peg. This, as you can imagine, is a reflection of his overall personality. Merton doesn't fit into cookie-cutter categories.
"It's a perpetual sound in my ears. It makes me ask myself if I've any right to personal happiness, any right to anything but to be as rich and overflowing, as smart and shining, as I can be made." (184.108.40.206).
Kate sometimes feels so bad about herself that she can't believe she deserves any happiness at all. When you have a dad as jerky and selfish as Lionel, it's difficult to think of your personal happiness as something that matters. Instead, Kate has been raised to believe that her worth lies in being a rich, entertaining hostess.
Never, he flattered himself, had he seen anything so gregariously ugly—operatively, ominously so cruel [as Aunt Maud's waiting room]." (220.127.116.11)
When he first goes to meet Maud Lowder, Merton Densher realizes that Maud is messing with him by keeping him waiting in her sitting room. While he sits there, Merton can't help but think that the room he's sitting in is some kind of torture chamber. His nervousness affects the way he views everything around him.
Mrs. Stringham was never to forget—for the moment had not faded, nor the infinitely fine vibration it set up in any degree ceased—her own first sight of the striking apparition, then unheralded and unexplained: the slim, constantly pale, delicately haggard, anomalously, agreeably angular young person." (18.104.22.168)
When she first met Milly, Susan found the girl kind of awkward looking and pale. But the more she got to know her, the more Susan realized (like everyone else in the book) that Milly is one of the most delicate and angelic-looking people ever to walk the earth. From this point in the novel onwards, Milly is never described as anything but beautiful, beautiful, beautiful. This first description, though, goes to show how much talking to Milly can completely change the way people see her. She's beautiful inside and out.
[He] would have affected her as the most intellectual person present if he had not affected her as the most frivolous. The latter quality was rather in his look than in anything else, though he constantly wore his double eyeglasses, which was, much more, Bostonian and thoughtful." (22.214.171.124)
Milly has a tough time pinning down Lord Mark. He seems to be this mishmash of thoughtfulness and shallowness. His baldness and double eyeglasses make him look thoughtful, but he also has a childish glint in his eyes. This quote establishes Milly as a person who is pretty good at judging books by their covers.
His large, settled face, though firm, was not, as she had thought at first, hard; he looked, in the oddest manner, to her fancy, half like a general and half like a bishop, and she was soon sure that, within some such handsome range, what it would show her would be what was good, what was best for her." (126.96.36.199)
At first glance, the great Sir Luke seems to have a stern face. But when she looks more closely, Milly sees that his face isn't hard, but sternly gentle. He has the look of a very dignified and respectable man. And, lo and behold, Sir Luke is a dignified and respectable man. What you see is what you get with good ol' Sir Luke.