Some TMNT stuff was always meant for grownups.
Renet was a teenage apprentive to Lord Simultaneous. Curious and impatient as she was, she did not care very much for studying. Renet first met the Turtles fooling around, when one day out of boredom she snatched her master's Time Scepter and was caught by him. She replied immediately and time-traveled off with Lord Simultaneous' Sceptre of the Sands of Time to Earth—more specifically to 1986 New York City. Renet and the Turtles became friends, but Simultaneous appeared and sent Renet into a panic. Fleeing Simultaneous further, she and the Turtles went to 1406 A.D., straight into a castle siege.
The castle belonged to the dark mage Savanti Romero, a former, but exiled student of Simultaneous, and the besiegers were the soldiers of Cerebus the Aardvark, who was after some scrolls, that Romero had stolen. Romero also managed to grab the Time Scepter, and the Turtles and Renet had to contend with Cerebus' help to attack the castle and get the Scepter back from Savanti Romero. The force was, however, no match for Romero's magic, but before it came to the worst, Simultaneous appeared. Romero attacked Lord Simultaneous who took the Time Scepter again, and cast Savanti into prehistoric times. He set everything right by giving the scrolls to Cerebus and returning the Turtles to New York City in the 20th Century. He punished Renet by making her dust his huge library for one week. When Renet muttered "Butthead", this was overheard by Simultaneous, he sentenced her to an additional week of chores for her insolence. (TMNT Vol. 1, #8).
Later, however, managed Renet to concentrate on her exam, and she gained over time—often with bitter experience—more responsibility for her role and more importance tasks to achieve.
In the non-canon Guest Era story Turtles Take Time, Renet later sent the Turtles a time cube borrowed from Lord Simultaneous. The Turtles, out of curiosity and boredom, used the cube to go on a trip to various eras, including the creation of the universe itself before returning to the present.
When the Turtles accepted Savanti's challenge in the present, Renet appeared and accompanied them to do battle with Savanti Romero in prehistoric times. After defeating him, they were forced to spend several weeks there until they were able to retrieve the Sceptre of Time (Tales of the TMNT #7, Turtle Soup #3 & Shell Shock).
In the non-canon Guest Era story Juliet's Revenge, thousands of years later—according to her personal calendar—a much older Renet, returned as a full-time champion, now the Mistress of Time in Lord Simultaneous’ place after his retirement, came back to give her friends the Turtles’ assistance against Savanti Juliet, who was attempting to exact revenge on the Turtles and their friends, for the apparent death of her husband by their hand. Renet used Juliet’s powers against her, killing her.
Renet, somewhat younger than the version of herself that helped them defeat Savanti Juliet but older than the version that accompanied the Turtles to prehistoric times, returned to send and accompany the Turtles to Feudal Japan to stop Savanti Romero’s latest plot of revenge. As Lord Simultaneous had once done, she exiled Savanti back to prehistoric times (TMNT Vol. 1, #46 & 47). In Dark Shadows.
Renet explains that if Raphael proceeds to teach Shadow Jones martial arts, this is the likeliest outcome. If he chooses not to, then the likeliest outcome is that she will be murdered by a Foot Mystic at age 23. Raph doesn’t like either option, and Renet explains that this is the very reason why looking in on your own future is never a good idea. Raph thanks her for the favor anyway, and Renet tells him that she owed him for saving her from a t-rex.
Returning to the present day, Raph finds a goth, teenage Shadow Jones beginning her martial arts training with Casey Jones. Raph knows he can’t talk her out of it, but asks that she call him "Sensei" from now on.
Elsewhere, back at Renet's place, she’s approached by Lord Simultaneous, who tries to tell her why she should never show other people their futures. Depressed, Renet says she’s figured the lesson out the hard way and will never let herself forget.
Renet can be seen watching an blind, elderly Donatello in the Tales of the Teenage Mutant Ninja Turtles TPB frontispieces and epilogue. Don solemnly climbs into bed, completely unaware of Renet’s presence, the Timestress begins to weep. Renet then activates her time scepter and vanishes.
Renet was also a guest at Splinter's funeral and later cleared up—though with obvious reluctance—April's true origins.
Tales of TMNT Vol.2 # 71
In the alternative history comic The Paradox of Chudnovsky Renet and Savanti Romero were actually a couple, but their personal differences about the objectives of their studies under Lord Simultaneous ultimately led to their separation. Savanti was not cursed by simultaneous, but physically corrupted by his investigation of the death-time. At the end of their struggle against Savanti, Renet is struck by his charm and her intelligence and mental maturity are restored to the state in which they had been entering the space-time university.
Renet's first IDW appearance is in the Teenage Mutant Ninja Turtles 2014 Annual, where she "invites" the IDW Turtles to the interdimensional Battle Nexus tournament. The cruel Councilor Nieli hosted a series of merciless gladiator fights, which only he really liked. Renet was also against the games for personal motives, because one of the champions, the warrior Baltizar, had won her heart. For this reason, they brought the Turtles to the hub of the dimensions and helped the culmination of these events to instigate an uprising against Nieli, which meant that Nieli was banished and the games were transformed into a far more peaceful affair.
- Her last name is given as "Tilley" in Transdimensional Teenage Mutant Ninja Turtles, but is never corroborated anywhere else. The RPG appendix Transdimensional TMNT also gives Renet's age as 16.
- 1986 January 1 Teenage Mutant Ninja Turtles (Mirage Comics) issue 8
- 1988 December 1 The Collected Gizmo has a story with her and Gizmo
- 1989 January 1 Teenage Mutant Ninja Turtles Graphic Novel book 3 issue 3
- 1989 April 1 Tales of the Teenage Mutant Ninja Turtles issue 7 "The Return of Savanti Romero!"
- 1989 December Shell Shock introduces Meanwhile... 1,000,000 B.C.
- 1989 December (and later, November 2007) the Tales of the Teenage Mutant Ninja Turtles TPB before/after
- 1990 June Turtles Take Time has a note from Renet
- 1991 December 1 Teenage Mutant Ninja Turtles (Mirage Comics) issue 42
- 1992 January Turtle Soup (vol. 2) 3 introduces "Sweat, Sweat, Sweet Renet"
- 1992 April 1 Teenage Mutant Ninja Turtles (Mirage Comics) issue 46
- 1992 May 1 Teenage Mutant Ninja Turtles (Mirage Comics) issue 47
- 2003 August 1 TMNT: Teenage Mutant Ninja Turtles issue 11
- 2004 February 1 TMNT: Teenage Mutant Ninja Turtles issue 14
- 2005 June 1 TMNT: Teenage Mutant Ninja Turtles issue 22
- 2008 October 1 Tales of the TMNT issue 51 "Night of the Living Gingerbread"
- 2010 April 27 Tales of the TMNT issue 69 "Dark Shadows"