I think the best character development I've seen in an RP so far is either Hope Everheart, because she went from
"omg no Alois and I are just friends. I'm scared of everything oh... Ciel... hi... we aren't that close yet.... I'll never tell my cousin and uncle what happened to me, they'd be mad..."
To
"You see this handsome blond prince? That's my fiancé! Rosalyn is my mother and I cannot be convinced other wise *turns to Will* hello father. This is Little Brat- I mean Ciel. He is my younger brother. Ew. Don't stand so close to me. I won't tell my uncle or cousin cause they'd kill a bitch... not that I'm opposed to that."

Or Marquis de Lafayette's character development from
"I'm straight and love my wife."
To
"*speaking to Washington and/or Mulligan* hi Daddy."