Namor was created in 1939.

Namor the Submariner I don't know who was the first Marvel character, but Namor was the first mutant, not the first character.

Namor, also known as Namor the Sub-Mariner, is a Marvel Comics character who is a mutant hybrid of Atlantean and human heritage. He possesses superhuman strength, the ability to fly, and the power to breathe underwater. Namor is often portrayed as an anti-hero, with a complex personality and a turbulent relationship with other superheroes such as the Avengers and the Fantastic Four.

Namor (full name Namor McKenzie) is also known as the Sub-mariner, originally introduced in Marvel Comics#1 in 1939.

In his fictonal history, Namor was a half human/half alantian mutant who fought with Captian America and the Invaders during World War II. He is later discovered by the Fantastic Four in contemporary times as a homeless amnesiac man, who finally regained his memory and returned to the sea to rule Atlantis as their king.

Attuma and General Krang are his biggest foes I would say, in that order.