He shoots the Dursleys a very dark look.

"S'pose I ought to try explaining what's going on, then," he says quietly.  "How much do you know about your parents?"

The Dursleys are very mad at the both of them and Harriet's very scared; she shoots them a nervous look.

The man shoots them a - less nervous - look.

"Not gonna let them do anything to yeh," he says.  "Couldn't if they tried.  Throw 'em out in the rain, if you like."

"...They're my guardians."

He sighs sadly through his nose.  "If I have anything to say about it - "

He stops.

"They shouldn't be, 's my opinion," he says gruffly.  "Wager we could find you some better ones, if you like."

"...Kids don't pick parents."

"Your aunt and uncle have been mistreatin' yeh," he says.  "Keeping secrets from you, keeping you locked in a cupboard.  This letter - "

He draws a letter, identical to the ones she's been receiving en masse for weeks, from one of his pockets.  (He's so big he has to hold it between thumb and forefinger.)

" - is your acceptance letter to Hogwarts School of Witchcraft and Wizardry.  I came here to make sure you got to read it, and to take you shopping for school supplies if you want to come, and to make sure you don't have to spend another day in this house if you're not safe here."

This is kind of overwhelming.

How is she supposed to evaluate what she wants.

He looks between her and the Dursleys, and sighs again.

”Why don’t you come outside with me,” he says, gentle but firm, and stands up.  “We can talk properly away from this lot.”

She nods, slowly, even as the Dursleys object.

He waves off their objections effortlessly; and once they’re out in the rain, he raps his umbrella sharply on the door he bashed down, which springs off the ground and snaps back into place.  The miserable rainy spit of rock that the hut sits on is mostly as she remembers it, except there's an extra rowboat docked in the miserable little pier.

“Shouldn’t let you hold this, strictly speaking,” he says, gesturing with the umbrella.  “It’s got - er - well.”

He trails off.  Fiddles with it for a moment; it pops open, and he stoops a bit and holds it out for her.

”Just stand under here,” he says.  “Go ahead.”

She steps under the umbrella.

"What does the letter say?" she asks, hesitantly.

He does - something - to the letter, like flipping a coin with his thumb, and the envelope pops right open, the letter coming to hover in front of her face.  There's important-looking letterhead, which reads:

HOGWARTS SCHOOL of WITCHCRAFT and WIZARDRY

Headmaster: Albus Dumbledore
(Order of Merlin, First Class, Grand Sorc., Chf. Warlock,
Supreme Mugwump, International Confed. of Wizards)

and has a faded sort of coat-of-arms framing it, a four-part shield with icons of a lion, serpent, badger, and eagle around an ornate letter H.

The letter itself says:

Dear Ms. Potter,

We are pleased to inform you that you have been accepted at Hogwarts School of Witchcraft and Wizardry. Please find enclosed a list of all necessary books and equipment.
Term begins on 1 September. We await your owl by no later than 31 July.

Yours sincerely,

Deputy Headmistress Minerva McGonagall

"Potter was your dad's name," the man said.  "James Potter."

"But my name's Evans."

"That's from yer mum," he says.  "Fine old wizard, Lily Evans.  We'll get that put right when we get to Hogwarts."

"Okay..."

"What's Hogwarts?"

"Best school of magic in the world," the man says.  "It's a place where young wizards like you go to learn to cast spells - and fly on brooms and brew potions and so on.  You live there during the school year, like in a Muggle boarding school."

"What about in the summer?"

"Folk without anyone to look after 'em can spend summers at Hogwarts too," he says.  "And if I have anything to say about it, you'll be one of 'em."

Small, and quietly: "...Okay."

He smiles gently.

"If there's anything in the house you need to get before we go," he says, "I can let you back in, or get it for you myself.  Otherwise... I say we head off now."

"I don't have anything..."

"Oh - the cake?"

" - forgot the ruddy cake!" he exclaims, and leaps to his feet.

He's in and out in a flash, the cake-box held delicately between two fingers in his free hand.  He holds it out for her.