The truth, however, was that he was a Marine Commander, working undercover in order to stop Doflamingo.
Rosinante was a tall man, as a 10-year old Trafalgar Law was not able to reach his waist. He was seen wearing a dark red hood from which two heart-like shapes hang from, a pair of beige jeans, a dark purple feather mantle much like his brother and a white shirt with pink hearts printed all over it. He also wears mismatched shoes. His blond hair was longer than his brother's as it came close to his eyes. Rosinante had tattoos (or makeup) over his eyes and mouth. He was also seen wearing sunglasses, though he did away with these upon his six-months journey with Law.
During his childhood, Rosinante had his hair in an unkempt fashion, covering his eyes. Prior to leaving Mariejois, he wore the typical garb of the Celestial Dragons, though his hair was not long enough to be permed the same style as his older compatriots. After leaving, his wardrobe consisted of simple yet fine clothes with a frilled cravat that became very ragged due to the uneasy lifestyle he had to endure. Suffering from torture at the hands of the locals who hated the Celestial Dragons, as well as other battles, left scars and stitches all over his body.
When Trafalgar Law first met with Rosinante, Rosinante was noted for hating kids, especially when Baby 5 and Buffalo played practical jokes on him, such as making him slip upon passing through the doorway or feeding him scalding tea that burned his tongue, resulting in his abusive behavior towards them. In fact, the first thing that Rosinante did to Law was to grab him and throw him out the window. This was actually part of an act, however, as it was his duty as a Marine to dissuade children from joining the crew, which he failed with Baby 5, Buffalo and Dellinger.
Despite his outward behavior of a violent man who dislikes children, he genuinely inherited his father's kindness (something Doflamingo himself admits) believing that his elder brother is a monster born evil and only acted crude to children in the hopes of driving them away from following Doflamingo. He cared deeply for his birth family as he wept for his mother's death and cried when his brother was shot with an arrow. From his kindness, he saw it his duty to stop his brother's madness, a sense of justice that led him to join the Marines as an undercover spy. Because of this, Rosinante was shown to be forgiving since he did not hold any grudge against Law for stabbing him in the back and even kept quiet about his transgression. When Law revealed his true name, he showed genuine concern and worry, even warning him about the danger that he was in. He was capable of compassion as he went as far as to go out of his way with both his brother and the Marines to take Law throughout the Grand Line to find a doctor to cure the boy's disease and flew into violent rages whenever the doctors cave into prejudice on their incorrect belief that Law was a walking contagious disease who survived the extermination of Flevance. Despite his extreme methods due to his empathy and courage he appears to follow his own ideals instead of adhering to the Marines' doctrine of Absolute Justice.
He was believed to be incapable of speech due to a prior accident and, according to Diamante, was unintelligent and cannot concentrate. However, this was merely a façade and he was fully capable of speaking, as well as perfectly intelligent, playing on how the others perceive him. Upon hearing Law's full name, knowing the importance of the initial "D.", he was forced to break his silence, showing more perception than what his comrades believed.
Although his mutism and dislike for children were mere acts to hide his true self, the one attribute that was genuine was his clumsiness, easily falling for Baby and Buffalo's pranks and constantly tripping himself. He was also seen smoking, though due to his clumsiness, he accidentally set his own cowl aflame every time he tried to light a cigarette.
Being born in the Donquixote Family, Rosinante was a World Noble, thus granting him political rights and privileges to do almost anything he pleased, as well as near limitless wealth and the ability to summon a Marine Admiral to strike down anyone who offended him. When his father forfeited the status of a World Noble for the family, he lost such privileges.
As a former executive officer of the Donquixote Pirates, occupying the Heart seat, Rosinante was undoubtedly a powerful fighter to attain such a rank. He was second-in-command of the many other officers and soldiers of the crew, alongside the other executive officers, and just under Doflamingo himself. His body has scars and stitches riddled over, showing great endurance and durability from withstanding the torture from angry civilians even as a child, and simply sat unflinching for a while after being stabbed deep in the back by Law before tending to the wound later that day. He managed to move and sneak away with Law even after having been shot multiple times and brutally beaten by Vergo, although Vergo made it clear that he was not trying to kill them.
As a Marine Commander he had command over all lower ranking soldiers, but he may have had more authority as he was a double agent who reported directly to an admiral.
Rosinante was noted to be unintelligent and lacking in concentration as well as being very clumsy as he fell for both of Buffalo and Baby 5's pranks and setting his own cape on fire while trying to light a cigarette. However, Diamante stated he was nevertheless talented enough to become an officer, possibly due to his lineage.[10] As with his mutism, it was an act that shows his power of deception, as Rosinante is actually intelligent to deceive his crew on by hiding his true self and Devil Fruit powers in order to spy on them, as well as being aware that anyone with the D initial should not be anywhere near Doflamingo. However he was shown to be clumsy anyways, as it is his nature.