It's mentioned in the TMNT:Ultimate visual history book.
On page 181 Brandon Auman says- "The Turtles definitely grow, but you don't want them to change all that much. You don't want Raph to lose his hot temper or Mikey to suddenly understand why he's acting like a goof. That said, we discover things about them that will broaden their characters for sure. Like why Raph is hot tempered, and how that ties into the crack in his shell...that kind of thing."
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