Both of them are voiced by Melissa Joan Hart.
Delia and Deidre Dennis, better known as Dee Dee, are thrill-seeking delinquents who inherited their grandmother's fascination with the Joker, and joined his namesake gang for kicks, and possibly personal gain through street crime.
Although they are two individuals, as twin sisters, they appear to act as though they are one person in two bodies, often speaking as one or completing each other's sentences, along with the fact they are co-referred to as Dee Dee. In fact, they always address each other as Dee Dee, and the way they speak to the other sounds very similar to a person talking to him or herself. They do care for each other and are willing to abandon a job to help the other, as during their first encounter with Batman, one broke away from helping to carry a large piece of technology to save her sister from falling to her death.
Besides their trouble-making ways, they are also definitively cruel and deceitful, shown when they pretended to be afraid of Batman (Terry McGinnis) and viciously attacked him the moment he turned his back on them, as well as how they had no qualms harming the elderly Bruce Wayne.
In the alternate timeline, they proved themselves to be murderous, having killed members of the Justice League.
Despite their impressive teamwork by acting as one person, they are still susceptible to sibling rivalry, as Terry tricked them into fighting each other. Moreover, they are followers, not leaders, shown when they cowed in the presence of the Joker, as well as Chronos.
Batman Beyond: Return of the Joker
Dee Dee, also called the Deeds along with Bonk, Chucko, and Ghoul, were recruited by the resurrected Joker for his plan to attack Gotham City. On his orders, the Jokerz stole advanced technology from a series of warehouses.
In the gang's first confrontation with Batman, the young Dark Knight made the mistake of taking them for inconceivable and useless girl "helpers". As soon as he turned his back on them however, they attacked with an amazing blend of skill, ferocity, and coordination.
Appearing as their "normal selves" all dressed in black dress and with their long blonde hair out, they later attacked Terry in a nightclub, and he barely managed to avoid them.
In the Jokerz' last confrontation at the Jolly Jack Candy Factory, Batman fought smart: first he goaded them using his invisibility powers, lured them into walking over a floor slippery with jawbreakers, before tricking them into charging into each other, knocking themselves out.
The twins were arrested. They were bailed out into Harley's custody while awaiting their trial, as Harley ranted about what disappointments they were to her.
Justice League Unlimited
In an alternate timeline, Dee Dee and the other Jokerz were recruited by Chronos as enforcers, and upgraded with radically advanced technology and weapons, giving them actual superpowers. The Dee Dee Twins have the ability to double whenever they come in physical contact with anything. In this time-stream, Dee Dee reportedly killed Kai-Ro. In another battle with the Justice League Unlimited, they succeeded in killing Terry as well.
- Melissa Joan Hart also played villainesses Anirbas and Katrina Spellman from Sabrina, the Teenage Witch, and portrayed a villainous version of Sabrina Spellman in the Funny Or Die video, Sabrina and Salem: Together Again.
- Melissa Joan Hart previously appeared in the 1996 film Twisted Desire as the evil Jennifer Stanton.