As the Witch of Envy, Satella was involved with Echidna and the other witches' deaths. Unlike the other witches who only despise the Witch of Envy, Echidna also hates Satella as well.
Echidna claims to have hated Emilia from the moment she first saw her. During her trial where Emilia was forced to face her past, Echidna made several derogatory remarks at the half-elf, mocking her for her dependence on Subaru. Echidna also seems to have some knowledge about Emilia's parents, claiming that she is as just as stubborn as her mother. Echidna adds she personally knows her real mother and it is one of the reasons for her hatred.
Echidna is greatly interested in Subaru's Return by Death ability, as it allows him to collect information from multiple points in time. To that end, she invited him to her tea party so in order to strike a conversation with him and later offered Subaru her assistance in the form of a contract. However, Subaru rejected the offer after the other witches gave hints that Echidna wasn't telling the full truth.
While she was alive, Echidna took Roswaal as her apprentice. After her death, Roswaal became obsessed with reviving her.
Unlike the rest of world, Echidna seems at least be acquainted Pandora, describing her to be the Witch of Vainglory to Emilia during her trial. However, she is dismissive of Pandora's power, claiming that it made her an expert at saving her own skin.