Blackfire is the older, evil, and sociopathic sister of Starfire. She looks just like her and will use it to destroy her sister.
Slade Strikes Back
She made her minor role debut when she joined Joker to destroy her planet, encountering her sister and Bender. Starfire fought her while Bender looked for and thrashed Joker to the infamous "where's my money?" beat down. She returned to help Joker kill the heroes but was defeated
The V Team Island Adventure
She returned to help Uka Uka against Bender, Starfire, and others, but she lost again after Uka Uka's defeat wasn't heard or seen from again.
New Team Ironhide
Blackfire is part of Megatron Prime's group.
Mervamon and Ponydramon
Blackfire was still evil for a time, but turned around and joined Team Ironhide.
She was killed by Alternate Doofenschmirtz, who fed her to Marceline's Dad and then used her evil power to corrupt Starfire in the future to make her an evil villainess. Blackfire was one of the few of Joker's forces not to be killed by Bender alongside Chameleon, Chuckles, Hinkle, and Noid
The Joker, Uka Uka, Brother Blood, Flexo
Starfire, Bender, Skipper, Marceline, Team Ironhide