What is a historical connection?

What's the historical connection between Jason Bourne and Nikki Parsons in Bourne Ultimatum?

• Without any/many spoilers, what did Nikki mean by "you were especially difficult for me"? What's her connection with Bourne?

• Answer:

  She had been someone who assessed his mental health while he had worked as a spy. She had expressed concern about him prior to his losing his memory and he had remembered her when he first started digging around. In the Bourne Identity, he had seen her and remembered her and that she had been kind to him which was one reason why he did not kill her. In the Bourne Supremacy after he came back to go after them for trying to kill him and having killed his girlfriend, he had seen her in the office and said he would only speak to her. She was the one who told him that he had been implicated through fingerprints at the scene of a failed operation. She knew his struggles and that he only wanted to be left alone but they kept on going after him. She had sympathy for him