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Tear Jerker: Kirby of the Stars
Kirby of the Stars had a few that come to mind, the first being 'Birth? Kirby's Little Brother' when the (robotic) dog exploded. Even worse than that is when Kirby is sitting on the beach that night, looking almost as though he's unable to comprehend its death. Than he sees it as a constellation, running away from him.
The death of Chilly the snowman in 'Goodbye, Snowman Chilly!' and the near death of the baby Galbo in 'Kirby's Mysterious Egg. It was implied Chilly would eventually return, but you still couldn't help but shed a tear at his apparent demise.
(Japanese) Episode #51, anybody? Sure, the ending's upbeat, but Kirby still runs away from Pupupu Village after he's led to believe nobody likes him, and then cries—yes, CRIES, tears and all—after remembering the good ol' days with his friends he thinks have abandoned him.
The funeral scene in Episode #93. The original Japanese version was even sadder because of the music.