Ramona Nowitzki was a very strange character, to say the least. She becomes an assistant of sorts for Sheldon Cooper, but she has a humongous crush on him. But he eventually discards her when she asks for credit for something she legitimately helped him on.
She appears out of nowhere many seasons later with blonde hair. It becomes very obvious that she still has a thing for Sheldon Cooper.
For which he is oblivious about, but it becomes obvious to everyone as she tries to wiggle her way into his life.
Despite fully knowing that he was in a relationship with Amy Farrah Fowler, she kisses him unexpectedly. Which makes him properly leave the office to find Amy Farrah Fowler and pop the question to her. When his new fiancee finds out what Romona did, Amy Farrah Fowler actually thanks her for it.