Realism, Tragedy, Quest
At its core, Realism was a literary movement that reacted against fabulous, unrealistic stories that tried to make everything look sunny and nice. (We're looking at you, Romanticism.)
Think of Realism as a grizzled old sailor spouting wisdom from the end of the bar; saying stuff like, "Argh. Life be hard." Realism tells it like it is. Realism doesn't sugarcoat. Realism is a bare-knuckle boxer with a huge scar on his nose. You can blame Realism for the times in this book where you find the story really slow or dry, because Henry James won't compromise on realism for the sake of speeding up the plot.
Over and over, you'll get pages-long descriptions of people's thought processes that don't seem to advance the plot at all. On top of that, you might find the characters not all that likable. But at the end of the day, The Wings of the Dove portrays the stuff of life.
And you know what's a big part of life? Tragedy. There. We said it.
We're not talking about how sad it is that Milly dies. Don't get us wrong—it's super sad. But what we talk about when we talk about tragedy is a hero falling from grace. Capital-t Tragedy is often about the person who doesn't kick the bucket.
In this case, Kate is our Tragic Hero(ine). She's sad but okay at the beginning of the novel. She may have a d-bag dad, but she has a loving boyfriend and a stern-but-loving aunt. But instead of confronting them head-on, she decides to be scheming and creepy, and this ends up with her having compromised her morals and perhaps having lost her main man, Merton.
If Kate is our Tragic Hero, then Merton is our questing dude. Check out our analysis in the Booker's Seven Basic Plots section and see why, in detail, The Wings of The Dove is a quest. But here's the quick and dirty version: Merton is sent to get some treasure (Milly's inheritance) and has to travel across borders and overcome obstacles to get it.
But, you know, because this book is also as Realist as a punch to the nose, Merton gets his treasure but also ends up with a shattered conscience and the feeling that most of humanity is f'ed up.