Kirby claimed that in the creation of the Fantastic Four, his concepts came first. His daughter Susan testified that her father showed her concept sketches of three of the four characters, one named Susan Storm after the then 16-year-old. In the FF, Sue had a teenaged younger brother Johnny, so they both started as young people.
Lee’s primary involvement, aside from being Kirby’s “conduit” to make the presentations to Martin Goodman, was scheming how to get his name on the thing as “writer” (he did it by signing every splash and creating an after-the-fact plot outline). For the purpose of this discussion we’ll say there was a story conference to determine how to repurpose a monster story and incorporate it into Kirby’s team concept. The story in question was about a scientist and his daughter, a nuclear explosion subplot that was later deleted, and a villain called the Mole Man. Lee’s contribution was to rewrite the scientist character as the leader (writing himself as Reed Richards) and to make the Sue Storm character the love interest. Imagine Kirby explaining Lee’s edit to his daughter.
Throughout his FF run, Kirby often had Sue Storm saving the day. As Kate Willaert showed on her Kirby Without Words blog, Lee undermined this at every turn, rewriting stories through the added dialogue, turning the heroine into the fashion-and-shopping-loving damsel in distress. Kirby wrote powerful women: Barda, based on his wife Roz, Sue Storm based on his daughter. Lee rewrote Sue as Joan Lee.