It's your brother Alan. Right now, he's staying in the other bedroom in the apartment so that you can save on rent. Your roommate, except he isn't sleeping in the same room as you.
You reply, "I'm busy!"
Your water girlfriend doesn't say anything and there's no more knocking. You have to leave the bathroom at some point but it's not clear what you're supposed to do. You can't leave the water girl in the tub to get flushed down the drain, if that's even possible. Maybe you can hide her under the sink, if she can flow into a small space?
She has managed to grab a towel which she draped around herself. It somehow stays dry, just as your fingers stayed dry until you exerted pressure. You're still thinking about how to hide this... until she opens the door before you can stop her. And of course, your brother had just walked up to the door again and she's right in front of him. He gasps. "You're... You're... what are you? You're transparent... like you're made of water."
You sigh. Well, looks like you're explaining it. "I found a magic shop. Real Harry Potter stuff. And you'd never believe it, except that you're looking right at magic and there's no way you can ignore it. They sold me a bargain item, a magic gun which turns anything into your girlfriend. So I used it on water." The water girl has her arm affectionately around you and is smiling.
"I'd say that's impossible... but I won't."
You explain, "It was a bargain because they discontinued it. It does what it says, but it may be hard to refill the charges in the gun. It has settings but I don't know what they are and it has conditions for the target to change back but I don't know what those are either."
Alan says, "Uh... brother's girlfriend, do you have a name? Can you speak?"
She makes a sound, but it isn't something you can understand. She makes another sound which sounds a bit less growly, but still isn't speech you can understand.
"I think," says Alan, "that she's trying to speak. If you just used magic, she's never spoken, right?"
The water girl nods and tries two more times. After that she says "I am trying" but it sounds almost like a man's voice. One last try and she finally makes a sound that is something like you'd expect from a girl's voice. "I guess I can speak. But I don't have a name."
You try to think of names. Do you want to give her a clever name or a normal name? Finally you say "How about 'Brooke'?"
She smiles and says "That's a lovely name," ignoring the joke. You and Brooke sit down in the living room. She doesn't make the furniture wet, although she accidentally hit the edge of a cabinet once and there was a tiny splash of water. Though she can talk, it seems she's reluctant to talk much. She certainly couldn't talk about TV shows she's watched, or people she'd met, or anything else that assumed she's been around for a while. Or could she? She has instant knowledge of English, she could have instant knowledge of other things.
Alan says, "If there's a condition for her to change back, she'll be gone then. Did you want to name someone who's going to just be a pool of water someday?" At that, an expression of sadness crossed Brooke's face.