Renet is apprentice to a "Time Lord" Lord Simultaneous. Bored with her life she decides to steal her master's time sceptre and travels to New York where she meets The Turtles. Lord Simultaneous follows her and demands his sceptre back and tells Renet that she will be punished. The Turtles decide to defend Renet against her presumed attacker. When Lord Simultaneous threatens the Turtles Renet uses the sceptre to transport all five of them to another place and time. They end up in the medieval Europe in 1406 ad. After meeting Aardvark Cerebus they are attacked by Savanti Romero who defeats them and steals the sceptre. The Turtles and Renet try to retrieve the scepre with the help of Cerebus, but Romero is too powerful and captures them all. Lord Simultaneous finally finds his student and defeats Romero with his enhanced time piece (a watch). Simultaneous takes Renet with him, gives Cerebus the scrolls he desired, sends Romero to Jurassic period and Turtles back to their own time.
Second appearance with the Turtles
Next time Renet meets the TMNT in "Tales of Teenage Mutant Ninja Turtles 7". In that comic book, she "helps" the Teenage Mutant Ninja Turtles to defeat Savanti Romero again. In that comic book Romero threatens to kill the Turtles if they do not confront him in the past (See more accurate description on the event from Savanti Romeros page).
4Kids has made cartoons out of both of the above mentioned comic book appearances. First story is TMNT 2003 season 3 episode 17 and second is in two parts in season 4 episodes 20 and 21. Stories are bit modified.