Vecna is Henry Creel, also known as Hawkins Lab test subject 001, an orderly who looked after Eleven, as revealed to viewers of season 4, volume 1. Eleven accidentally sent him to the Upside Down, where he came into contact with the power of the Mind Flayer, after he ultimately rebelled against Brenner and murdered the other upgraded kids. After a BTS video of Vecna drinking Starbucks surfaced on the official Stranger Things website, actor Jamie Campbell Bower, who plays Vecna, became a meme. Vecna's monstrous appearance is a prosthetic costume worn by Bower.
Bower discusses the viral meme and his opinions on Vecna's surprising popularity outside of the Upside Down in an exclusive chat with Screen Rant. Bower replies that he has seen the meme but isn't sure what he thinks when asked if he has seen it and what his reaction is. See the complete quotation below: I have indeed seen it. [Laughs] I'm not sure. I was always on the fence. No, I'll take the fact that people have remarked it makes me seem more human.