My secret is that Ned Marcus is my pen name. Marcus was the name my mother nearly gave me at birth, and it was one she sometimes used. So it was a natural choice of pen name because I already felt familiar and comfortable with the name. Ned, I just liked.
Actually, it's not really a big secret. I tell people I meet in person, it's just something I've not thought about talking about online before.
By the way, the images are for fun only. I don't look like that, even though I based the second image on a real photo.
The main reason for choosing a pen name was that my name was taken—and extremely common. My name is Mark Chapman and, unfortunately, a notorious murderer has a similar name. He has David in his name. I still have people ask me why I killed John Lennon, and occasionally people become aggressive. I now understand stories I've read about actors playing villains getting attacked in public because some people can't separate reality from fantasy. A quick online search will also find hundreds of pages of him, but apart from him, Mark Chapman is a very common name.
There used to be a Facebook group for Mark Chapman's with tens of thousands of members, if I remember correctly. A well known British sports commentator is called Mark Chapman. Also, there is a fantasy/SF writer called Mark Chapman, as well as a writer on Anglicanism with the same name.
It's too much.
As Mark Chapman, I run a niche ELT company teaching interview skills to marketing professionals—I also work as a freelance copywriter.
If you meet me, feel free to call me Ned or Mark. At writers' conferences, I go as Ned; elsewhere, I go as Mark.