Mr Compress, whose real name is Atsuhiro Sako, is a major antagonist affiliated with the League of Villains and later the Paranormal Liberation Front in My Hero Academia. He operates as a theatrical thief and illusionist, and his villain name derives from his Quirk, Compress, which allows him to shrink any object or person he touches into a small, marble-sized sphere. This ability can also be applied to parts of his own body or to environmental elements such as fire or debris, and he can reverse the effect instantly by snapping his fingers. The Quirk factor resides in his arms, and after losing his left arm in battle with Overhaul, he can only use Compress through his remaining right arm.

Mr Compress comes from a notable lineage. He is the great-great-grandson of Oji Harima, a legendary figure known as the Peerless Thief who lived near the dawn of hero society. Harima despised corrupt heroes who used their position for personal gain and instead stole from the wealthy to give to the vulnerable. This legacy of anti-heroic justice was passed down through generations, and Mr Compress grew up idolizing his ancestor and believing in the same principles of exposing unfairness and fighting corruption. He viewed the League of Villains as the only group actively working toward the kind of societal reform his ancestor would have supported, which motivated him to join their cause.

In terms of personality, Mr Compress is a showman at heart. He loves to command attention, be flashy, and make every event entertaining. He is talkative, humorous, and arrogant toward both allies and enemies, often monologuing and showing off his entertainer skills. Despite this flamboyant exterior, he is cold, calculating, and highly intelligent, often serving as the brains of the League of Villains and handling on-site tactics, communication, and extraction of allies. He is unflinchingly loyal to his comrades and willing to sacrifice himself for their cause.

His role in the story begins during the attack on U.A. High School's summer training camp, where he participates as part of the Vanguard Action Squad. He plays a key role in the kidnapping of Katsuki Bakugo, using his Quirk to compress and abduct the target. Later, he becomes a right-hand operational support member within the League, separate from Kurogiri, and continues to serve as a key operative in various missions. During the Paranormal Liberation War, he makes a desperate sacrifice to free his teammates from Best Jeanist's binding threads by using his Quirk to carve out a portion of his own flesh, compressing himself and others to allow their escape. This act proves fatal, and he dies from the self-inflicted injury, considering it his final performance and a fulfillment of his family's legacy.

Key relationships include his deep bond with the rest of the League of Villains, particularly Spinner, whom he thanks before his sacrifice. He respects leader Tomura Shigaraki and is committed to the League's anti-establishment goals. His interactions with heroes are marked by his theatrical style and tactical precision. Development occurs gradually as his mysterious background is unveiled, shifting from merely a flamboyant thief to a character driven by a noble family tradition of challenging injustice, ultimately culminating in his selfless end. Notable abilities extend beyond his Quirk to include expert stage magic, trickery, stealth, and quick improvisation, making him a versatile and dangerous opponent despite the loss of one arm.