His personality is sadistic as shown when he takes pleasure in experimenting on the likes of children, he also demonstrates tendencies of a superiority complex when Monkey D. Luffy mentioned his Gasu Gasu no Mi had no effect on his immune system due to Magellan's Doku Doku no Mi, both being poisonous devil fruits of differentiating class, and Caesar becomes rapidly aggravated in the fact of Luffy comparing his logia powers to that of a mere paramecia user.
Caesar was first seen after Franky and his group broke out of their cell. As he examined the hole that Franky made, he commented that only Vegapunk's laser was capable of such a feat. When he heard Trafalgar Law was fighting the Marines from G-5, Caesar was displeased since the plan was to chase them away. He was then shocked to be informed that the Marines saw the kidnapped children and feared that his secret activities would be exposed. Monet then arrives and informs him that Vegapunk's dragon and Brownbeard's centaur patrol unit were defeated. He then learns of the Straw Hat Pirates' presence on the island.
After Law deals with the G-5 Marines and returns to the facility, Caesar berates the Shichibukai and demands an explanation for his actions. Law replies that he is the one who should be complaining.
He soon contacts a person named Joker who gives him permission to kill the Straw Hats, the G-5 Marines, and Kinemon while Joker will make it look like an accident at sea. One of the centaurs defeated by Luffy's group then come to Caesar, telling him that Brownbeard has been captured and begs him to help. Caesar tells him that he has a medicine ready and that he will heal him and the other centaurs. However, instead, he locks the centaur in a chamber and exposes him to a seemingly new gas he developed. Caesar asks him for certain symptoms, like nausea and hallucination, and the centaur appears to have them. Caesar is next seen pressing on a button, and a explosion-like sound is heard as he purged the chamber along with the loyal centaur inside, and in the next panel, smoke coming out from a shaking exhaust pipe is seen.
When Monet asks whether his experiment failed, he claims none of his experiments ever fail, even during the "failed" experiment four years ago where his weapon of mass destruction killed a massive amount of people. He adds that he had even followed up and did surveillance on all the victims of that experiment, documenting the effects as proof of his scientific mastery. He euphorically asks the World Government (though not directly) where the failure is and tells them that he will show them who the number one scientist is.
Law later gives Smoker's heart to Caesar. Caesar holds Smoker's heart in his hand, while Law looks on silently with a displeased look in his eyes and a frown. Caesar is in good spirits thanks to the capture of Smoker's heart (likely planning experiments with it) and says that he already sent soldiers to deal with the Marines and that he knows what the outcome will be. Law asks what happened to Straw Hat, and Caesar reveals that having taken Monet's warning about the Straw Hats' possible increase in strength seriously, he sent two powerful people after them: the Hitmen of the Snowy Mountain, the Yeti Cool Brothers. It was later revealed that he also told them to kill Brownbeard and capture Franky because he possessed the technology of Vegapunk's laser.
When Monet informs Caesar about Law's association with Luffy at Sabaody Archipelago and Marineford, Caesar pulls a pistol on Law and questions him about his relationship to the notable pirate. Law denies having anything to do with the Straw Hats' presence on the island. Caesar then realizes that Law wouldn't betray him, as he wouldn't make things more complicated for them, after becoming a Shichibukai and coming to Punk Hazard. As Caesar is about to leave, Law asks him why the kids would come back, even when left alone. Caesar then proudly presents him a NHC10-pill and tells Law that he gave them to the kids, much to Law's disgust. As Law is leaving, Caesar points out that people who think too much are hard to handle.
- Caesar was the title for the Roman emperors, derived from the cognomen of Julius Caesar.
- Like Magellan, Caesar's sinister face, wild hair and horns make him resemble a stereotypical image of a demon; particularly Baphomet, a pagan deity-turned-devil most commonly depicted as a humanoid goat.
- In his first appearances his coat did not show "GAS" written twice on the front as it does in Chapter 666 and Chapter 671.
- He is also, so far, the only Devil Fruit user who seems to keep his Logia Devil Fruit active all the time, giving him his gaseous appearance.
- He is the second Devil Fruit user to alter the properties of his own fruit's power using artificial chemicals, the first being Tony Tony Chopper with the Rumble Balls.
- Caesar's Japanese Voice Actor, Ryūsei Nakao is largely known from his previous roles in other Anime such as Mayuri Kurotsuchi from Bleach and in One Piece: Strong World as Indigo. Oddly enough, all three individuals have similar behavior and position (crazy, sadistic scientists).
- In Chapter 663, his name is spelled as "Caesar Clown" which takes place of the hiragana, as seen Smoker's monologue. However, in Volume 68's character introduction, it is spelled as "Caesar Crown".