Decimaru is a character from the anime Me and the Alien MuMu. He is a cat-like alien and a fellow citizen of the show's titular character, MuMu. Information about his specific history before arriving on Earth is limited, but it is known that he is part of the same alien race as MuMu, a species trying to recover its lost technology by learning from Earth's civilization. Unlike MuMu, who focuses on studying home appliances, Decimaru appears to be more interested in learning about Earth's public facilities and infrastructure.

In terms of personality, Decimaru is initially characterized by a haughty and condescending attitude toward humans. He is described as holding a hostile view of Earthlings, whom he refers to as inferior beings. This aloof and prideful demeanor is a key aspect of his personality, often leading to conflict or sarcastic remarks. Despite this exterior, he is not a purely antagonistic figure; his behavior also carries a certain feline charm and a somewhat cute or endearing quality that prevents him from being entirely dislikable. He displays very distinct "catitude," meaning his actions and mannerisms are authentically feline even when he is being highly anthropomorphic.

Decimaru's core motivation is tied to the central goal of his species. He is on Earth for the same overarching purpose as MuMu, which is to observe and learn from human technology in order to revive his home world's lost civilization. His specific emphasis on studying public works suggests a more systematic or large-scale approach compared to MuMu's hands-on tinkering with electronics. This mission places him in direct contact with humans, whom he must reluctantly rely on despite his professed disdain for them.

In the story, Decimaru serves as a secondary alien character who complicates the dynamic between MuMu, the protagonist Sakurako Umeyashiki, and the human supporting cast. He is not the main antagonist but rather a rival and a foil to MuMu, representing a different attitude toward the mission and toward humanity. His initial alignment is notably with the human character Akihiro Tsurumi. The circumstances of this partnership are unusual; Decimaru was originally attempting to infiltrate the home of Akihiro's father, who is a government minister, but ended up being taken in by Akihiro himself. This backstory explains much of his standoffish and irritable behavior toward Akihiro, as he finds himself dependent on someone he initially sought to exploit.

Key relationships heavily define Decimaru's role. His relationship with Akihiro Tsurumi is central to his character arc. Akihiro is a self-described "mechanical idiot" who is handsome and loves animals, especially cats. There is a one-sided dynamic where Akihiro is very affectionate toward Decimaru, who is often cold and dismissive in return. However, Decimaru grows to have a notable soft spot for Sakurako Umeyashiki, the main human protagonist. He develops a clear crush on her, becoming visibly jealous when Akihiro or other male characters get too close to her. This creates an interesting pseudo-love triangle between Sakurako, Akihiro, and Decimaru, with the twist that Akihiro seems to show genuine affection for both Sakurako and the cat-like alien. Decimaru's interactions with other alien characters, such as MuMu and a fellow alien named Siberia, are also important, as they form their own community and plot strategies together regarding their mission on Earth.

As the series progresses, Decimaru shows signs of development, primarily through his softening stance toward specific humans. While he maintains a generally superior attitude, his affection for Sakurako and his begrudging tolerance of Akihiro hint at a growing emotional complexity. He becomes more integrated into the human group's activities, participating in meetings and adventures rather than remaining a distant observer. He is frequently involved in the comedic scenarios that unfold, such as being discovered napping on a robot vacuum cleaner in a cafe, which blends his alien nature with very Earthly, cat-like instincts.

Regarding notable abilities, Decimaru shares the fundamental traits of his alien species. He is a bipedal, talking cat who is highly intelligent and capable of understanding and analyzing human society. His primary skill seems to be observation and infiltration, given his initial plan to enter a minister's home. While not overtly a combatant, his alien nature implies capabilities beyond a normal Earth cat. His most prominent ability, however, might simply be his powerful and persuasive feline charm, which he uses unknowingly to endear himself to humans like Akihiro, even when his behavior is less than friendly.