Koutarou Onodera, known by the nickname Ono D, is a supporting character in the anime Full Metal Panic? Fumoffu. He is a second-year student at Jindai High School and shares a class with Kaname Chidori and Sousuke Sagara. He has a tall, athletic build, brown eyes, and short hair dyed blond, which fits his place on the school basketball team.

Ono D has a lighthearted and somewhat frivolous personality, and he serves as the mood maker of his class, keeping the atmosphere around him lively and casual. His easygoing nature makes him a natural part of the circle of classmates who surround the main characters, and his ordinary, sociable behavior contrasts with the serious and unconventional conduct of Sousuke. The story does not explore deep personal ambitions for him; his motivations are those of a typical high school student enjoying school life, friendship, and sports.

In the story, he begins as a minor extra who occasionally delivers a line or two. As Fumoffu progresses, his role becomes larger, and toward the end of the series an episode places him at the center of events, marking his growth from a background presence into a more prominent member of the supporting cast. His most notable ability is his athleticism, especially in basketball. He also later appears as a supporting character in Full Metal Panic! Invisible Victory.