G.Callen

He is the strangest agent of NCIS. Callen has a life crisis: he doesn't knowm anything about his past or his family and not even his first name! There are episodes dedicated only to the past life of Callen for example: "Tin Soldiers" where he find a little led soldier in his sister's grave that someone had gave him when he was a kid and he goes to find more about it. He is the partener and friend of agent Sam Hanna, he speaks russian and has contacts inside KGB including the former agent Arkady, who has helped the team in some episodes.