JoshJosh is Deputy White House Chief of Staff in the Josiah Bartlet administration. Josh's character has one of the sharpest minds on the President's staff; he is a witty, somewhat cocky, boyishly charming know-it-all.
DonnaDonna is Senior Assistant to Deputy White House Chief of Staff Josh Lyman (or, as she jokingly calls herself in one episode, the " Deputy Deputy Chief of Staff"). Although all of the senior staff's assistants are continuing characters with personal backgrounds, Donna is the most well-defined and frequently featured staff member on the assistant level.
Josh and Donna
They have not only a close working relationship, but a close personal friendship as well. Since the first episode of the show up until the middle of Season 6, when Donna quit her assistant's job, the pair constantly flirted and bantered with one another.