If that is the case, then I understand Hotpaint to a degree. Having a character with a callsign that does not embody his personality, or just relate to some other part of him, either physically or psychologically, would be odd. As for creating another character, I've worked something up that I hope will work.
Name: First Sergeant Sam Garner
Age: 20
Height: 180 cm
Weight: 84 kg
Appearance: Sam has dark brown hair, full, deep-set green eyes and bushy eyebrows that follow the natural curve of his round forehead. His face is oblong with a strong, set jaw, his nose is broad and rounded, and his lips are thin and always split in a cheeky, friendly smile. His hair has been cut very short, revealing larger than average ears. Generally he has a well-proportioned head.
His body is lean and muscular with taut muscles stretching like wires underneath his golden-brown skin.
Callsign: "Mapcap" or "Lefty"
Personality: Sam is an uncomplicated guy whose general demeanor is amiable. He rarely gets on somebody's nerves or becomes agitated by others; he likes to keep a neutral and seemingly indifferent attitude toward conflicts. He seems to just focus on his job as a soldier. He is bold and reckless, sometimes putting his life on the line when it could be avoided. He seems to cope well with the constant pressure of war, but whether his behavior is just a defensive mechanism is not clear. He is fiercely loyal to his comrades, and gets his tasks done. He is among some of the youngest soldiers in the militia, and tries to live up to the expectations of his seniors, but he is exposed to error due to his young age.
I have two callsigns in mind for this character, so you can choose between either one.
Madcap he would have gotten because of his sometimes reckless behavior. Lefty he would have gotten because he has gotten his left ringfinger shot off on during combat.