Diesel (ディーゼル Dīzeru ) is a small, sharp-tongued and sentient brown creature with two long appendages and a yellow eyepiece (he lost his eye in an accident), capable of flight with a hovercraft which he pilots. He is affiliated with Samus Aran and appears in Samus and Joey and Metroid EX. He works as a junk dealer at the Junk Quixote.
In the actual story, Diesel does not appear until chapter 4 of volume 1. He works as a car mechanic in a junk shop on a massive unnamed space station. Samus visits the station to leave Joey in his care, only to find Jealousy in the shop, and Diesel being held captive in a spherical container bound by a chain to the ceiling. Jealousy states that he stole Diesel's inventory and summons Bomad to combat Samus, who refuses to use her weapons for fear of hurting Diesel, while Joey uses the opportunity to rescue him. He mistakes him for Diesel's pet. Joey temporarily uses Diesel in his container as a shield when ambushed by Pirates, and he survives the detonation of Bomad's gravity bomb. Samus laughs as she reveals to Joey that Diesel himself was the hostage, and not his pet. Samus asks Diesel to take Joey under his supervision, which he gladly obliges to, seeing as he has a chance to work the boy until he "dies".
Diesel accompanies Joey to Degrade, where the Special Forces Ship Otowa carrying his mother has crashed. With the help of Samus, they are successful in rescuing the survivors, which includes Dr. Apronika. After this, Diesel and Joey return to the Junk Quixote. Sometime later, Joey befriends a robot he dubs U-ton, but soon discovers that it destroyed a ship and killed all of its passengers. Diesel follows Samus and Joey on their pursuit of Greed, who steals Samus' abilities and gives them to six entities called the Deadly Six Stars. Their mission eventually takes them back to Jatsuk, where Diesel has not been for a long time. He explains he left after his species stopped making machinery, which they only did because their technology was used in warfare. Diesel tries to convince them that their technological power is their pride, and capable of being used for good. They return to their machine-making ways after he solves the riddle of how to disable a bomb, by making it into a new mechanism instead of simply dismantling it. They create a rocket out of it, and shoot down Blast's ship. Samus kills him and restores her Bomb.
Chapter 12 implies he is aware of Samus' true gender. The author Idzuki Kouji confirmed he is the only character in the manga who knows her true identity, but has never seen her face.
"Samus' friend from planet Jatsuk."
"The galaxy's best junk dealer. Samus asked him to take charge of Joey."
"Samus's friend. Runs a junk shop. Samus asked him to look after Joey."