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Tear Jerker: The Kaceystar Trilogy
Kacey's arm being pulled off by Starscream and the surgery afterwards. The moment when Optimus begins crying because he feels like a failure as a father for not being able to protect her is really heartwrenching...
When it appears that Elita has died and Optimus goes into a fury and almost kills himself.
My Beautiful Rising Star
When Kacey runs away during a blizzard because she feels unloved.