David Kalen, A.K.A D.K (Not to be confused with Donkey Kong!) is my favorite minor Spider-Man villain, only appearing in 3 issues, with his first appearance being Spectacular Spider-Man #230 when Ben Reilly took the mantle for awhile
His backstory involves him and his brother Henry investigating toxic chemicals dumped into the environment by a businessman named Mr. Sanders. When checking out a barrel in a lake, a piece of explosives were attached to it, causing an explosion with Henry being killed and David surviving while being mutated by the toxic chemicals. Fuelled with rage, He sets out to get revenge on Sanders for both his brother's death and for killing the environment. David can transform into a purple sludge-like monster with the ability to melt and decompose anything he touches.
He encounters Spider-Man as he tries to reason with him that Sander's death won't solve anything and eventually D.K stops himself from killing the corrupt businessman as he's sent to jail and D.K goes to Ravencroft Asylum to seek therapy for survivor's guilt and depression
After a few years when Peter Parker returns, Ashley Kafka calls him in to see D.K as he's slowly killing himself with his own toxins from the inside because of the loss of his brother unaware that Peter has never met him, nor know anything about D.K other than the encounter Ben Reilly had with him. D.K suffers a mental break down and attempts to break out of the asylum and attack Spider-Man. They both try talking things out about the loss of their own brothers and D.K is sent back for more treatment, only even more depressed and dying even more
His final appearance was in an issue where Spidey battles a younger Vulture, where D.K escapes the asylum, meets up with Spider-Man as he battles Adrian and thanks him for trying to help him and be there when he could before decomposing and dying, all while trapping the Vulture with his tentacles and somehow ageing him back up to his current age
Another reason i want a figure of D.K is because this was the first comic i ever read as a kid and his origin reminds me of my little brother who passed away when i was 7. I don't have many memories of him and when i do i often fall into depression because i miss being raised with a sibling and having someone to spend time and play with growing up, so i kinda relate to D.K in that sense.
I doubt Hasbro will make a figure of him given he's only had 3 appearances and isn't that much different from other shape shifting liquid characters Marvel has, but i'd be the only person on the planet that would be over excited to get an action figure of him!