This version of the Joker wasn't the mass murdering psychopath of the comics, but a goofy, mischievous prankster who relied on harmless weapons (sneezing powder-squirting flowers, non-lethal joy buzzers, etc) and Vaudeville-type humor to commit his crimes. One of his first goals was to unmask Batman and publicly expose his nemesis' identity. Some of his crimes were for little more than his own harmless amusement, while others were far more dangerous. Therefore, his threat level depends on the episode.
In the film, Joker teamed up with Penguin, Catwoman and Riddler to form the United Underworld, to disband the United Nations. His role in the Combination Trap (that was devised by the Riddler) was the Jack-in-the-Box to send Batman flying toward the Penguin's exploding octopus; however, one of the unnamed thugs fell onto the jack in the box and was killed by the exploding octopus. The four super criminals managed to dehydrate the nine members of the United World Security Council and take them to the submarine, however Batman and Robin draw it to the surface, where the Dynamic Duo fight the United Underworld. After taking several beatings from Batman, Robin fights the Joker and defeats him by punching him over the edge, into the water. He is captured along with the other criminals when Batman and Robin chain them to the side of the submarine and Batman calls the coast guard where the police can take them to prison.
His many schemes after this included committing crimes based on the signs of Zodiac, stealing the famous Renaissance art collection for ransom and trying to take over Gotham City with a flying saucer. He can also easily persuade women to help him in his crimes with his charm and wits.
Even though Batman defeated him many times, the Joker vowed never to rest until the last laugh was his. Although not being as fixated as with previous and current versions, this Joker still enjoys fighting Batman and likely wants to be the one who defeats him.
This is, by far, one of the few versions of the Joker to not be depicted as horrible and completely monstrous without redeeming qualities but more as a lighthearted troublemaker (unsurprisingly, since the 1960s series was intended for a young audience). A notable difference between Cesar Romero's Joker and his mainstream counterpart is that Romero's Joker (due to the show’s censorship policies) never committed murder (his goons did mention he dumped their parole officer into Gotham river in one episode but it’s never stated or reported the guard died leaving it likely the guard swam to safety wholeness Jokers goons escaped), while the mainstream Joker has committed many murders.
He did try to kill both Batman and Robin and in various episodes but showed some moral standards as well such as making sure his henchmen safely pumped poisonous gas out of a death trap to make sure civilians wouldn’t get killed by the gas.
However, the mainstream Joker was already a dangerous foe like in today's comics before the 1960s series was produced. When the Comics Code Authority was established, one of its policies involved the banning of extreme violence, toning down the Joker into a goofy, thieving trickster whose appearances became rare in the early 1960s due editor Julius Schwartz's dislike of the character, but Romero's portrayal in the show became popular enough to bring back the character, who then turned more dangerous and evil in post-1960s appearances.
Cesar Romero refused to shave his mustache for the part, resulting in the distinctive look of his very visible facial hair covered in white makeup.
The Joker's appearance resembles his appearance from the Golden Age comics, though his suit looks a little pink.
This Joker is also used as a headline thumbnail for pages who have been headlined.