harry potter - Who is Gilderoy Lockhart based on?

J.K. Rowling has stated that Gilderoy Lockhart is the only character in the Harry Potter series who is based on a real person she knows.

Are any of your characters based on real people?

The only character who is deliberately based on a real person is Gilderoy Lockhart. [Laughter]. Maybe he is not the one that you would think of, but I have to say that the living model was worse. [Laughter]. He was a shocker! The lies that he told about adventures that he’d had, things he’d done and impressive acts that he had committed… He was a shocking man. I can say this quite freely because he will never in a million years dream that he is Gilderoy Lockhart. I am always frightened that he is going to turn up one day. He is just one of those people from your past whom you feel you have never quite shaken off. I will look up one day at a signing and he will say, “Hello, Jo”. [Laughter]. Other people have contributed the odd characteristic, such as a nose, to a character, but the only character who I sat down and thought that I would base on someone is Gilderoy Lockhart. It made up for having to endure him for two solid years.

_{J.K. Rowling at the Edinburgh Book Festival, in response to a question from the audience, through Accio Quote.}

I've read some speculation that it is her ex-husband, Jorge Arantes.

(...) Jorge Arantes, a dashing journalism student three years her junior.

(...)

Maria Ines Augiar believed Jorge to be both possessive and jealous, while Steve Cassidy, who ran the school where Rowling worked, viewed him as rough and untrustworthy.

_{"The JK Rowling story", The Scotsman, June 16th, 2003.}

They were married from October 1992 to November 1993, while having met only a couple of months before their wedding, so that's a year and a half at best, not the two years J.K. said she had to endure in her answer. But other characteristics seem to match.
And Hermione, who is generally accepted as an author insertion, does have a crush on Lockhart before he is exposed.

But

She told how the Daily Express wrongly claimed she based "unpleasant" Potter character Gilderoy Lockhart on her ex-husband Jorge Arantes. It was based on a former flatmate.

_{"JK Rowling tells Leveson inquiry of her struggles with fame", Daily Mirror, November 25th, 2011}

So who is it?

Answer

Definitely not her husband. All we know is that they were once flatmates.

This is J.K. Rowling’s witness statement to the Leveson Inquiry (an inquiry into the standards of the British press) which the Mirror is referring to:

Defamation: private life

The press have also published false allegations related to my private life. In 2004 the Daily Express published an article saying that one of the “slimiest” characters in Harry Potter was based on my ex-husband and that I had admitted basing arrogant wizard, Gilderoy Lockhart on him as an act of revenge. It was even stated in some of the articles that I was prepared to abuse my position as an author in order to humiliate my ex-husband by making this revelation to a national newspaper. [...] It was remarkable to see the Daily Express changing their headline throughout the course of the day as later editions that same day carried a different heading. This showed that they were aware that such allegations were totally false. [...] The article had been falsely based on a light-hearted talk I gave once to a group of children at the Edinburgh Book Festival. I did not reveal anyone’s true identity when discussing the inspiration for the character. An apology was printed.

Her full witness statement can be read on the Guardian website; this comes on page 30 of 33.

(Strictly speaking, the allegation is that she admitted basing Lockhart on her husband, not that she actually did.)

The Edinburgh Book Festival is presumably the same one you quoted in the question; I haven’t checked the publication date of the Express article.

Given that the Express retracted the claim, and she refuted it while under oath, we can assume it really wasn’t her husband.

As for the flatmate claim, the Huffington Post has a quote from the inquiry to back this up:

“I said humorously the character was Gilderoy Lockhart was based on someone I had lived with briefly,” she says. “That's true, but this person can’t probably even remember we were flatmates.”

I would not expect her to describe her ex-husband as somebody she lived with “briefly”, or as unlikely to remember that they lived together.

I’m not aware of any other details of who Lockhart was based on. For obvious reasons, she doesn’t talk about him much.