Post characters who are spies and assassins as well as secret/undercover agents

Meyrin from Black Butler

Zenzo Hattori was one of the top assassins of the oniwabanshu, now he is just a freelance ninja

Kisuke Urahara (Bleach)

he is a a former high-ranking member of the of the stealth force.......in other words assasin..........kisuke is the master of Assasin, that why he became the 3rd Seat of the 2nd Division, which was intertwined with the Onmitsukidō. As part of his duties as one of the top five Seated Officers, Urahara became head of the Onmitsukidō's Detention Unit. He has repeatedly shown impressive skill in staying hidden and unseen while maneuvering towards his target. He can completely hide his presence, remaining undetected until he intentionally reveals himself. During his battle against Yammy, he repeatedly caught his opponent completely off guard and effectively took advantage of it.[204] Even the perceptive and intuitive Aizen was caught by surprise.He was also able to launch a successful sneak attack on the Quincy Quilge, sensing when he wasn't using his Blut Vene for defend. this shows how much deadly Assassin kisuke is.......he he he he.........

Kabuto from Naruto was a spy for quite some time.

Train Heartnet in Black Cat is at the beginning of the anime

Dororo in Sgt . Frog is a natural spy and undercover agent . He also used to be an assassin but after meeting Koyuki he didn't want to kill anyone anymore .

Gajeel was a double agent for Fairy Tail and let Makarov keep an eye on his son's guild's activities.

Itachi Uchiha (Naruto) was an assassin and a spy/double agent.

Yamazaki Sagaru from Gintama is a spy.

yuuji kazami:
Hides it from his classmates

Yamazaki Susumu from Hakuouki and Peacemaker Kurogane was a spy. Same person in two different "history" anime about the Shinsengumi.