User can suppress or negate emotions in themselves or do not possess emotions at all, allowing them to ignore emotional distractions, suffering from psychological/emotional stress, and/or feeling from affecting their thinking-processes.
In some cases, the user is simply unable to feel emotions of any kind.
Akame (Akame Ga Kill!) has been conditioned to be a near-emotionless assassin who is notorious for killing her targets in cold blood.
Kurome (Akame Ga Kill!) has been conditioned to be a near-emotionless assassin who is notorious for killing her targets in cold blood.
Kazuo Kiriyama (Battle Royale) is completely emotionless due to brain damage, allowing him to operate at full efficiency in everything, and is considered a demon of emptiness.
Train Heartnet (Black Cat) was known for his heartlessness during his time as an Eraser for Chronos, killing his targets efficiently and mercilessly.
The Executive Militia (Bleach) of the Onmitsukidō are known for their cold-bloodedness to execute traitors.
Ulquiorra Cifer (Bleach) represents nihilism, and as such, shows no emotions.
With eyes like that of a reaper, Rei Ōgami (Code:Breaker) can kill anyone he deems evil in cold blood, ignoring all pleas and reasons.
Yukihina (Code:Breaker) remains stoic even when expressing his desire to avenge his friend's death.
Vegeta (Dragon Ball) showing the typical apathetic trait of Saiyans, easily disposing Nappa for losing his ability to fight despite their longtime comradeship.
Broly (Dragon Ball) crushing his father and his space pod for trying to abandon him on a dying planet, showing a cruel disregard for even a parent who raised him.
Mard Geer Tartarus (Fairy Tail) has long since suppressed his own emotions and believed that emotions are a hindrance.
Utsuro (Gintama) is apathetic and emotionless, as he considered his 500 years of immortality being an empty existence, and even the mind-reading Batou considers him to be "empty".
Kanna (InuYasha) is a demon of void, and has no emotions as a result.
When Shinichi Izumi (Kiseiju: Sei no Kakuritsu) had his cells fused with Migi to repair his heart and before the death of Reiko Tamura, he became distant, unsympathetic, and loses the ability to empathize with others, as he not only thinks but feels emotionless like a parasite.
Sasori (Naruto) is apathetic and cold due to lack of parental love, and his puppet body is a physical metaphor of an empty and emotionless vessel.
Sai (Naruto) has gone through Root's extensive mental condition and training, resulting in him being almost completely emotionless, until spending time with Naruto.
Sasuke Uchiha (Naruto) cruelly and apathetically piercing through Karin to kill Danzō to avenge his brother, Itachi's sufferings.
Obito Uchiha (Naruto) shows no sympathy for anyone due to his hatred of reality, causing the death of his mentor and violently attacking his former comrade Kakashi.
Kisame Hoshigaki (Naruto) is like many Kirigakure ninjas, being conditioned to abandon their emotional attachments by killing comrades, something Kisame is all so willing to do.
Madara Uchiha (Naruto) cruelly and apathetically kills his fellow clansman, Sasuke, for the sake of his own Eye of the Moon Plan.
When Bartholomew Kuma (One Piece) became a living weapon, he lost his free will and became a heartless "human weapon".
Seta Sōjirō (Rurouni Kenshin) giving his trademark false smile, hiding his emotions and rendering him unreadable.
After her dream demon was killed, Yumi (Yumekui Merry) lost all her hopes and dreams, leaving her an apathetic and emotionless shell.
Loco (Marchen Awakens Romance) suffered an unknown tragedy in her past which broke her spirit. As a result, her eyes and voice alike display no emotion.
Having been experimented on and abused for most of his life Dufort (Zatch Bell!) rarely displays emotion for the world he has come to hate.
Gowther (Nanatsu no Taizai) lose all his memories, feelings, and entire sense of self do to be cursed by the Selflessness commandment.
Iron Maiden (Valkyrie Crusade) being an automaton designed to kill, is completely emotionless.
Astaroth (Valkyrie Crusade) is completely immune to and unable of feel love.
Each incarnation of Master Xehanort (Kingdom Hearts) inherit the originals lack of empathy and selfishness towards the lives of others. It's this quality among other things that has kept him alive for soo long.
Agent 47 (Hitman) is the iconic hitman, with a supreme control over his emotions. It's this and his many skills, that make him one of the best assassins around.