"You know Seymour," said the girl, and crossed her legs again. "He says he doesn't want a lot of fools looking at his tattoo."
"He doesn't have any tattoo! Did he get one in the Army?"
"No, Mother. No, dear," said the girl, and stood up. (1.107-9)
Here, we suspect that Muriel actually does understand something about Seymour. It's perfectly clear to her that he's speaking metaphorically, while her mother is left in the dark.
She sat down on one of the made-up twin beds and – it was the fifth or sixth ring – picked up the phone. (1.2)
It sounds here like Muriel is selfish or at least self-centered. We have to wonder how this affects the way she deals with Seymour's madness.
"Well. In the first place, he said it was a perfect crime the Army released him from the hospital – my word of honor. He very definitely told your father there's a chance – a very great chance, he said – that Seymour may completely lose control of himself. My word of honor." (1.51)
Much of the tension of the story is introduced this way; we wonder if, indeed, Seymour will "lose control of himself" by the end of the story.
"Did he try any of that funny business with the trees?" (1.23)
Salinger very subtly informs his reader of Seymour's madness without giving us too many explicit details.
"He told him everything. At least, he said he did – you know your father. The trees. That business with the window. Those horrible things he said to Granny about her plans for passing away. What he did with all those lovely pictures from Bermuda – everything." (1.49)
Seymour seems to have a very different take on death than most people. It's likely that whatever he said to "Granny" about her "passing away" was not taken in the spirit it was intended.
Section II (Seymour on the Beach and in the Hotel)
Ask me something else, Sybil," he said. "That's a fine bathing suit you have on. If there's one thing I like, it's a blue bathing suit."
Sybil stared at him, then looked down at her protruding stomach. "This is a yellow," she said. "This is a yellow."
"It is? Come a little closer." Sybil took a step forward. "You're absolutely right. What a fool I am." (20.20-22)
At this point, we know that Seymour is mentally unstable because of Muriel's conversation with her mother. So all his comments to Sybil leave us guessing: is he really unstable, or just playfully joking with this little girl?
"Yes. Yes, I do," said the young man. "What I like particularly about her is that she never does anything mean to little dogs in the lobby of the hotel. That little toy bull that belongs to that lady from Canada, for instance. You probably won't believe this, but some little girls like to poke that little dog with balloon sticks. Sharon doesn't. She's never mean or unkind. That's why I like her so much."
Sybil was silent. (2.64-5)
Passages like this one can convince us of Seymour's sanity. He's clearly joking with Sybil, as any adult who is great with children might. He even takes an opportunity to reprimand her – if playfully – as an authoritative adult.
"I see you're looking at my feet," he said to her when the car was in motion.
"I beg your pardon?" said the woman.
"I said I see you're looking at my feet."
"I beg your pardon. I happened to be looking at the floor," said the woman, and faced the doors of the car. (2.2-5)
This is the first time that Seymour appears to us to be abnormal. What's so interesting is that his behavior hasn't changed at all since he was joking around with Sybil. While this behavior might be perfectly acceptable around children, it appears inappropriate in the adult world.
"My daddy's coming tomorrow on a nairiplane," Sybil said, kicking sand.
[…] "Well, it's about time he got here, your daddy. I've been expecting him hourly. Hourly." (2.17-18)
Seymour's nonsensical conversation has a place with Sybil – this is a perfectly acceptable, playful way to talk to children.
"See more glass," said Sybil Carpenter, who was staying at the hotel with her mother. "Did you see more glass?"