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Blue Diamond is a major antagonist turned supporting protagonist in Steven Universe. She is one of the Gem Matriarchs on Gem Homeworld and a member of the Great Diamond Authority ruling over Homeworld. Blue Diamond was apparently responsible for overseeing attempted colonization of Earth, and later, the attempts to quell the rebellion of the Crystal Gems.
She is voiced by Irish singer Lisa Hannigan.
Blue Diamond fell into depression following the apparent death of her sister Pink Diamond, before learning the later's death was faked.
- Kidnapping Greg for her Human Zoo, which she sees as an act of kindness to Greg due to her believing that the Cluster is still going to destroy the Earth.
- She wishes to atone for ignoring Pink Diamond's pleas to spare the Earth by preserving the later's legacy.
- She wants to keep humans in a nature preserve in memory of Pink Diamond.
- Even during the Rebellion, Blue Diamond is shown to have a respect for high-ranking members of her court, as she thanks Sapphire for her vision.
- She is perhaps the first Homeworld Gem to quickly realize that humans are capable of empathy and feeling loss, and she lifts her veil and lowers herself to Greg's eye level to speak with him as an equal.
- Her desire to preserve Pink Diamond's legacy includes keeping the humans in the Zoo well cared for and keeping the Earth Quartzes and other Rose Quartzes alive (even if the latter are bubbled), both of which Yellow Diamond finds absurd. She also intended to save Greg by kidnapping him and taking him to the Zoo.
- She keeps the trial going, wanting to hear Steven and his defense Zircon out, and won't execute Steven outright when he demands to be punished for Rose's crimes.
- When Yellow Diamond starts going ballistic on the Zircons when the Defense one implies that the Diamonds shattered Pink, Blue steps in on their behalf.
- She helps Steven and Connie rescue Garnet, Amethyst, and Pearl's gemstones after Steven told her the truth.
- She helps the Gems, Greg, and Connie to get Steven back to normal along with Yellow Diamond, White Diamond, Spinel, and the Cluster.
- "I Am My Monster" is the final episode to feature Blue Diamond.