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Why is the Greek god Hades considered evil?

He wasn't considered evil. The Greeks didn't define their gods as "good" or "evil". He was associated with darkness, which WE associate with evil.

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What symbol for the greek god Pluto?

Pluto is actually a Roman god, and he was the god of the underworld, two of his symbols were black horses and a black chariot.

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What is the symbol for Hemera the Greek god?

It is A Small Rose    Edit: Day itself is Hemera's symbol as she is the goddess of day;  also a crowning ball representing light, and chariot.

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