Lady Marchmain seems perfectly nice, doesn’t she? She’s pious, refined, concerned for her children’s well-being, and dealing quite wonderfully with the fact that her husband left her for another woman. So explain these reactions to her:
Anthony: "She […] keeps a small gang of enslaved and emaciated prisoners for her exclusive enjoyment. She sucks their blood. You can see the tooth-marks all over Adrian Porson's shoulders when he is bathing. […] He's bled dry; there's nothing left of him. There are five or six others of all ages and sexes, like wraiths following her round. They never escape once she's had her teeth into them. It is witchcraft. There's no other explanation."
Sebastian: "She really was a femme fatale, wasn’t she. She killed at a touch."
Anthony, again: "What a poor time that woman is having! It only shows there’s some justice in life."
Is any of this true? Are any of these rather extreme descriptions justified? We can answer only through Charles’s eyes, but let’s take a look anyway. As he spends more and more time with Lady Marchmain, Charles does indeed confirm her manipulative nature. "One was never summoned for a little talk," he says of her, "or consciously led to it; it merely happened, when she wished to speak intimately, that one found oneself alone with her." Even her requests for help in handling Sebastian are laced with this apparently super-human ability to manipulate: "she took hold of her subject in a feminine, flirtatious way, circling, approaching, retreating, feinting; she hovered over it like a butterfly; she played ‘grandmother's steps’ with it, getting nearer the real point imperceptibly while one's back was turned, standing rooted when she was observed" (1.5.291).
Still, Charles doesn’t write her off and officially side with Sebastian until he reads her little book on Uncle Ned. What’s up with that book, anyway? Lady Marchmain says of her dead brothers, "They were three splendid men; Ned was the best of them. He was the last to be killed, and when the telegram came, as I knew it would come, I thought: 'Now it's my son's turn to do what Ned can never do now.' I was alone then. He was just going to Eton. If you read Ned's book you'll understand." It’s not exactly clear from the text, but Lady Marchmain is trying to get Sebastian to take over the male responsibilities of her own family, since Ned (her last living brother) has now kicked the bucket. This isn’t at all a happy arrangement for the carefree and youthful Sebastian, who wants only to be happy. Charles finally understands her intentions after reading the book on the train, which explains this little exchange:
"Did you have a 'little talk' with Mummy?"
"Have you gone over to her side?"
The day before I would have said: "There aren't two sides"; that day I said, "No, I'm with you, Sebastian contra mundum."
OK, OK, so the woman is manipulative. But does that really explain all the hostility we see? No, not completely. Fortunately, Cordelia comes to our analytical rescue one again. (Boy, is she handy or what?): "When people wanted to hate God they hated mummy." This makes particular sense for Lady Marchmain’s children, who had Catholicism imposed upon them by their mother (and certainly not their father, who converted himself just to keep her happy). Since holiness means suffering, and Lady Marchmain is responsible for her children’s obligation to holiness, it follows that she is responsible for their suffering. We can start to see why Sebastian or even Julia so resent her. As for Lord Marchmain, that’s an entirely different story – and a different "Character Analysis."