Appearance (in a Marvel comic): Marvel Two-in-One #21 (November
Appearances: Marvel Two-in-One #21, Giant-Size Spider-Man #3.
Years Active: 1930s-?
Yeah, I know. He's not a Marvel Comics character. He's a pulp character, and shouldn't be here. They don't own the rights to him anymore. True, true, all true.
During that short period when Marvel had the rights to the character, he did appear, in-character, in-continuity, teaming up with the Thing against...well, some forgettable menace, anyhow, and fighting alongside Spider-Man against an extra-terrestrial villain. The appearances are in continuity and have never been disavowed, so Doc counts. I know Marvel can't do anything with him now, since they don't have the rights to the character, but he still counts, as far as I'm concerned.