Some TMNT stuff really isn't for little kids.
Shared between the the Mirage and IDW continuities (but thus far only portrayed in IDW), Renet's future self, also known as Lord Renet, represents the same individual after she has aged and become an elderly Time Lord. Her involvement is central to the plot of TMNT: Bebop & Rocksteady Destroy Everything!
When Savanti Romero attempted to steal Utrom technology during the Cretaceous era, the future Renet appeared to thwart him. She was interrupted by the attacks of Bebop and Rocksteady, which allowed Savanti to steal her Time Scepter. He used it to mortally wound her, and left her to die while he escaped back into the future.
The future Renet then encountered her past self, and cryptically told her to "get the scepter back... and remove the gem." With that, she activated her stasis beacon, causing her body to appear mummified.
Many millions of years later, the younger Renet used the older Renet's Time Scepter to remove the stasis beacon from her future self, healing her. The future Renet reassured her younger self that they had done everything right, said farewell to the Turtles, and departed through a time tunnel.