Prince Aechmea

Prince Aechmea is the main antagonist of Land of the Lustrous, though he is not introduced until later on. On the Gems' perspective, is the leader of the Lunarians and the one behind the series' bad events. However, by the time Phos gets to talk with him it's clear (from the Lunarian's perspective) it's actually Kongou-Sensei the one who is causing most concern in the story. That said, it's entirely possible Prince didn't tell Phos the entire truth regarding that matter.

Personality
Prince Aechmea is cunning, persuasive and composed, whereas being quite wary of the Lunarian tribe's eternal life. He views immortality of his tribe as a curse. He is desperate on making Adament-Sensei back to his side so that he could do prayers, allowing all of the Lunarians into afterlife as they ever wished.

However, in spite of his politeness as well as concern over his Lunarian kins, Aechmea is rather self-loathing, calling all of the Lunarians including himself as "ghosts of garbage." In addition, he is shown to be cruel and merciless to the Lustrous. To achieve this, he is prepared to go to great lengths. Over abducting gems and grinding them into dust, he displays no remorse, showing he can be very cruel. He also shows a wrathful side that makes him much more demonic in shape, shown during the night raid led by Bort and other gems that supports Adamant when he's angry with Cairngorm over it.

Aechmea is also a good liar, being able to persuade Phos, Cairngorm and other gems, who arrived onto the Moon with them (including Yellow Diamond and) to turn against Adamant and join his force, making the Lustrous killing each other. His cruelty is extended when he forced Cairngorm to remove the shard of Ghost Quartz away from their eye, so that Ghost would not mind-control Cairngorm into caring Phos.

It is also implied that the Prince of Lunarians, instead of trying to help them, is actually using Phos as a pawn just because of their strength and determination, but in case of Phos failed or betrayed him, he would take them down. It's amplified after the end of Night Raid, where Phos is badly injured. Aechmea sent Cairngorm, now fully dressed in Lunarian attire, to gather Phos, Padparadscha and Yellow while capturing other gems, even to the point of allowing Cairngorm to shatter Phos into pieces.

Trivia

 * The name Aechmea is a genus in the botanical family Bromeliaceae, which includes plants such as pineapples. The name comes from the Greek aichme (αιχμή, meaning 'a spear'), which may refer to the spears the Lunarians use to attack the Gems.