Arthur's sister, born in the second week of October, three weeks after he left for Solmere. He does not find out she exists until he walks into his own front room ten months later and notices somebody looking at him from a doorway.
How the chapter does it
She is not announced. Ch. 338 puts her in the room and lets Arthur work it out, and the archive keeps the sequence because the sequence is the joke and the joke is the point.
He turned his head toward the doorway that led back into the kitchen. There was nobody there. He looked at the doorway for two full seconds, decided he was tired, and turned back to the conversation [...] Then he looked at the doorway again. There was a child in it.
She was extremely small. She was standing, just about, with one hand flat against the doorframe for the entire purpose of remaining standing [...] and she was looking directly at Arthur with the total, unembarrassed, unblinking attention that only very small children and Mabel Ruck are capable of.
Not older than a year, with "the specific top-heavy wobble of a person who had been walking for a fortnight and was still not sold on it".
Arthur's response is to ask his father a question in the wrong register entirely:
ART: Was I the one standing in the way of your greatest desire? CAL: Hmm? ART: For fifteen years. Was I the impediment.
Why nobody told him
Wren knew before Arthur left in September and chose not to write it.
WRN: She was born the second week of October. Three weeks after you went. I knew before you left and I did not tell you and I have thought about that decision approximately four hundred times since, and I still think it was the right one, because you'd have spent your first term at that academy worrying about a thing four days away that you could not have helped with. ART: Mum- WRN: I'm aware it was also a bit cruel. Your father has raised it.
Calder had raised it, and goes on raising it, and Wren gives her own reason for the silence:
WRN: I said it wasn't cruel, it was thrifty, because the post west is fourpence and a face is free.
What she does
She examines him. That is the whole of her part and it is enough.
There was, in the way she was doing it, absolutely no recognition whatsoever, which was the part that got underneath his ribs and stayed there: this was a person who lived in his house, who had his mother's chin and something around the eyes that was unmistakably his father's, and who had never in her life seen him before and was working out what he was from first principles.
Her first word about him is not a word.
ART: Hello. I'm Arthur.
Nami considered this with enormous seriousness.
NAM: Ah. ART: Arthur. NAM: Ah-fuh. CAL: She's got it. ART: She has not got it. CAL: It's two more than she had this morning.
Then she settles it by putting "one entirely wet hand flat on Arthur's face, which appeared to be the conclusion of some process she had been conducting since the doorway", and says it again with more confidence, and Arthur cries about it in front of four people and does not, on balance, care.
Her tag is NAM, which makes her the youngest speaker the archive holds.
Afterwards
Ch. 339 has her dismantling a piece of bread on Elsie's lap and dropping it at the exact moment Calder finishes explaining what a closed sea means, so that "the sound it made was disproportionate". Ch. 340 has her asleep against Elsie's shoulder at supper.
Elsie takes her off Arthur's lap "without asking and without Nami objecting", which is the entry's evidence that Elsie has been in that house since long before Nami was in it. See Elsie Tanner.