Page "Donatello (Teenage Mutant Ninja Turtles)" Paragraph 3

Donatello seems to hold the turtles together and this was highlighted in the 2003 series during which he was once mutated ( again ) into a ferocious beast who was unable to recognise even the people he was closest to.

Without his knowledge, the turtles were forced to turn to two of their enemies, Agent Bishop and Baxter Stockman, for assistance.

Another example of this was when he was sent into an alternate reality where the Shredder ruled the world, Splinter and Casey were both dead and the turtles had fallen apart.

Don managed to reunite the future versions of his brothers and although all three of his siblings were killed, he defeated Shredder before being returned to his own time and preventing the events from ever happening.