Tweedle Dee is one of twin gatekeepers serving the Hatter Mafia faction in Wonderland, operating specifically at the Hatter Mansion. He and his twin brother, Tweedle Dum, are collectively nicknamed the "Bloody Twins" by non-Hatter allies due to their violent tendencies. They jointly guard territory under Blood Dupre's command, ordered to kill unauthorized individuals approaching the gate unless instructed otherwise. Before the series' events, Dee and Dum frequently ganged up on Pierce Villiers alongside Boris Airay, contributing to Pierce's eventual departure from the country to escape them.

Physically near-identical in their child forms with short black hair, Dee is distinguished by his blue eyes, contrasting Dum's red eyes. Dee wears a blue uniform mirroring Dum's red one, both featuring identical chains on the front. In black-and-white media, Dee's clover emblem appears on the right side of his clothing with the heart on the left, opposite to Dum's placement. When transforming into their adult forms, Dee's hair grows longer and is tied back with a blue ribbon, differing from Dum's shorter style held by red barrettes.

Their personalities blend childish spontaneity with cruelty. They display innocence in violence, describing killings as "playing" with victims. Preferring axes over guns because "axes are more fun," they skip work obligations to pursue violent amusement. Despite shared duties, they possess distinct individual preferences like favorite colors or ice cream flavors. Both exhibit talent for clever and rude comebacks. They view themselves as a single entity in attachment, falling in love with the same person simultaneously, yet claim they would fight to the death if that person desired only one. They show deference to those perceived as mature or beneficial, while insulting or teasing others indiscriminately.

Their relationship with Alice is complex. After an initial hostile encounter where they attempted to slit her throat, they develop a strong attachment, referring to her as "Onee-san" (translated variably as "Older Sister," "Big Sis," or "Lady"). This term persists even in adult form, potentially indicating a mafia-related honorific. Unlike their typical childish behavior towards others, their interactions with Alice lack childlike innocence. They are completely devoid of cooking ability. Within Wonderland's social structure, they maintain a friendship with Boris Airay from the Amusement Park territory and enjoy teasing fellow Hatter Mansion worker Elliot March.