Complex sentence: Mossad's presence includes one of the main characters, Agent Ziva David, who is a former Mossad Agent and was NCIS's liaison to Mossad until the end of Season 7, when she becomes a full-time NCIS agent.
Split into simpler sentences: Mossad's presence includes one of the main characters, Agent Ziva David, who is a former Mossad Agent. She originally filled the position of Mossad liaison to NCIS, until the end of Season 7, when she became a full-time NCIS agent.