They have the same appearance as the Gravity Pines twins, but the Gleeful twins make sure to take care of their looks. They're very attractive and swooning. They are usually dressed in black or blue. They both always wear their magic amulets; Dipper's in the form of a bolo tie and Mabel's in the form of a headband.
The Gleeful twins are very evil and are rather different. Sometimes, they don't get along, and they fight. But their fights are minor and don't cause much damage. They are more evil than Gideon, as the twins are willing to kill and won't show any remorse.
In their Tent of Telepathy, they perform major and minor magic tricks. Mabel is Dipper's assistant in his acts. With his sister, they will secretly kill/use people who 'volunteer' during a show. The one who volunteers will be given a challenge. While the audience is still watching, Dipper and Mabel will use their powers to make the volunteer look 'alive' by making the volunteer move and even when they are already dead.
To the public, they appear as sweet and polite young kids, but in reality they are young monsters. They enjoy murder and the pain of their victims. Sometimes, they poison their victims and wait until night. Usually, they will torture the victim. Then they transport them to another dimension via magic. They aim to be the most powerful people ever, aiming to collect all three journals (they currently own Journal #2), and practice all the magic in them.The only known people they are kind to are each other. The twins are different in personality, but they are best friends and partners in crime. Together, they make the perfect team in destruction. Sometimes, they don't get along, and they fight. But their fights are minor and don't cause much damage.
The twins also kidnapped Reverse Bill and kept him captive in a cage nobody can break.
Mabel and Dipper are very powerful and are theorized to be half-demon.
(And no, bill cipher didn't get their mom pregnant.)