"So who've you been beating up tonight? ...Another ten-year-old? I know you did Mark Evans two nights ago -"
-- Harry to Dudley and his gang (OP1)
Mark Evans was a boy who lived near Privet Drive, who, at age ten, was beaten up by fifteen-year-old Dudley Dursley (OP1).
In the search for clues, some sharp readers focused on Mark Evans because his name was the same as Petunia and Lily, and he was mentioned living in the same neighborhood where Harry was being protected from the Dark Lord. Readers thought he might be an important character. What if Little Whinging was part of Dumbledore's "witness protection program"? What if not only Lily's blood was protective, but her very name? What if Snape had a family hidden there too, and "Mark" was clever pun to thumb his nose at Voldemort? So many theories, so little time. When a question about Mark Evans won the FAQ Poll on JKR's Official Site, fans gave a sigh of relief because finally they might hear the truth, yet many were disappointed by the answer. For despite his familiar last name, Mark Evans was unrelated to Harry, or to anyone in Lily’s family, and he was NOT the Half-Blood Prince, or a long-lost cousin. While the author was amused by all the fan theories, his name was just a throw-away:
Then why – WHY – (I hear you cry) – did I give him the surname “Evans”? Well, believe me, you can't regret it more than I do right now. “Evans” is a common name; I didn't give it much thought; I wasn't even trying to set up another red herring. I could just as easily have called him 'Smith' or 'Jones' (or 'Black' or 'Thomas' or 'Brown', all of which would have got me into trouble too). (JKR (FAQ))
While the author never intended Mark Evans to become a red herring, or misleading clue, he actually became a perfect example of one.
From the Web
What is the significance, if any, of Mark Evans? from Rowling's original website on the Internet Archive