Jyushimatsu Matsuno's name derives from the Japanese term for the Bengalese finch, combining kanji for "pine" and characters for "fourteen." His position among the sextuplets was initially ambiguous, sometimes placing him as the youngest, but recent works confirm him as the second-youngest son.

Early portrayals depicted him as the quietest and gentlest brother. Later characterizations shifted dramatically, presenting him as the loudest, noisiest, and most talkative sibling with a talent for singing. These differences were later reconciled by describing his quiet personality coexisting with a strong love for singing.

As a child, Jyushimatsu closely resembled his brothers, occasionally linked to purple as a theme color. He displayed kindness and thoughtfulness, avoiding roughness and crying easily. Despite this, he joined in mischief like digging pitfalls or adding dandruff to shaved ice. His obedient nature made him gullible and easily manipulated, sometimes acting against his own interests. He felt insecure about his indistinctiveness among the sextuplets, perceiving his presence as the weakest and lamenting that his name was "never called first."

During his teenage years, particularly ages 16 to 18, Jyushimatsu adopted a delinquent persona marked by narrowed eyes and a bitter expression, though he reverted to his naturally happy demeanor alone. His appearance featured a single cowlick, an untucked uniform shirt, and an undone pants zipper. He attempted crude behaviors for distinction, such as pretending to smoke invisible cigarettes and exposing his buttocks. This phase was resolved through interactions with his adult self in later media.

As an adult, Jyushimatsu stands out with his perpetually wide-open smile showing his tongue and a single cowlick. He typically wears a yellow hoodie with stretched sleeves, blue swim shorts, white socks, and yellow slippers at home. Formally, he dresses in a light blue blazer, gray slacks, and black dress shoes like his brothers. His personality is the most childlike and eccentric of the sextuplets, radiating constant energy and excitement while lacking seriousness. His bizarre behavior makes social integration difficult, leading others to speculate about hidden darkness, though he is ultimately accepted as being in his own "genre."

He displays kindness, notably helping Ichimatsu find a lost cat. However, his anger, often triggered by cold or other stimuli, can manifest dangerously. He exhibits illogical actions, like annually attempting to capture Santa Claus by tying him up, and demonstrates reality-warping abilities such as spontaneous self-multiplication or muscle growth.

Jyushimatsu shares his closest bond with Ichimatsu, frequently pairing with him for mischief or tasks like guarding hideouts, reading manga, or fighting based on mistaken identities. He regularly interacts with Karamatsu, collaborating on schemes such as spying on their father or arguing over opportunities. His dynamic with Todomatsu involves shared reactions to family events, while with Choromatsu, it includes babysitting duties and joint efforts selling soba.

In the "Osomatsu-kun Grows Up" setting, Jyushimatsu becomes a doctor by age 35, sporting a Chaplin-style mustache. His professional competence is questioned when he fails to save his father from choking on a fish bone, leading to familial blame-shifting. His marital status remains unclear in this future.

Jyushimatsu's foreign names reflect cultural adaptations: "Songye Liusong" in Cantonese (meaning "Sixth Pine") and "Yuk Bindung" in Korean (meaning "hollow"), varying by dub version.