The Hulk debuted in Journey into Mystery. No, not really. It was Xemnu the Titan, who was also referred to as Hulk.
He’s an alien fugitive felon who lands on our planet and hypnotizes Earthlings into building him a spaceship, then leaves. People don’t remember him being here.
He returns four issues later.
And an earthling tricks him into hypnotizing himself in a mirror, which kills him. But he will later be integrated into the Marvel Universe.
It’s interesting to see that Marvel just didn’t pull this guy out of left field. As I recall ‘Marvel Premiere’#3, and ‘The Defenders’#12, he gave the original Defenders a pretty tough time. As I also recall this period of Marvel history, the Valkyrie was the company’s Number One pin-up girl! This would remain the case at least until Marvel Girl returned, and Storm debuted, in ‘Giant-Size X-Men’#1! Notable mentions: Dave Cockrum’s Scarlet Witch and Mantis, and the Black Widow in ‘Daredevil’ and ‘The Champions’! Just love those super-babes!!