Many characters, from heroes to villains, are created with the intention of having a role. For heroes, to be a recurring characters in the superhero world with titles or appearances in teams. For villains, to be a recurring antagonist throughout a heroes' rogues gallery or the MU.
But for some characters, it was very obvious they weren't intended to be anything special. These are characters who became far more prominent than they originally intended. I used "prominent" because this refers to the core comics that influence the surrounding media, as really you can say that any character became bigger than intended thanks to the MCU, which feels like a cop out.
To begin:
Baron Helmut Zemo
This actually inspired me to make this topic from looking back at the "Marvel characters who changed from their debut" thread. He was introduced as a one-off villain in Captain America #168 in 1973, created by Roy Thomas, Tony Isabella, and Sal Buscema, under the guise of the Phoenix (not to be confused with X-Men). He lacked many of his characteristics, instead being a man driven by revenge for the death of his father, Baron Heinrich Zemo, at the hands of Captain America. He died at the end of the issue by falling into a vat of Adhesive X, and that was that. Just a villain meant to be used once, who would tie into Cap's old and deceased nemesis, and never appear again.
Until he appeared again in Captain America #275 in 1982, over 100 issues and nine years later, where he became the new Baron Zemo. He literally had no appearances between his first, all the way until then. It was only after this point where much of his defining characteristics came into the fold such as the calculating mastermind, being Cap's secondary arch-nemesis, leading the Masters of Evil, the purple mask, being sympathetic while still a villain, the "Prussian aristocrat" motif, and other aspects like his master swordsmanship and expertise in firearms.
I'd say J.M. DeMatteis (writer of the Captain America titles which returned him), Mike Zeck (artist who collaborated with DeMatteis on the title and created his iconic look), Roger Stern (writer of the Avengers, including the definitive Under Siege storyline for Zemo), and later, Kurt Busiek (creator of the Thunderbolts, which is the other definitive Zemo story) did far more for the character than Thomas/Isabella/Buscema who simply created a one-off baddie and then forgot about him. Hell, you really can say they created Helmut Zemo, but not Baron Zemo.
So basically, he was introduced as a one-off character who died at the end of his appearance, and then became a prominent Cap and Avengers villain much later while being a very popular bad guy.
Anyone else to note?