The Core

The Core is a major antagonist of Amphibia, serving as a minor antagonist in Season 2 and the main antagonist of Season 3.

It is an artificial being and the true master of King Andrias. It was created long ago and is an amalgam of the smartest minds from across Amphibia's history. The Core lived under the Newtopia Castle in a secret basement. Later, its conscience was transported to Marcy Wu's body through a helmet, leaving the deactivated mechanical body behind and creating a new identity, Darcy.

Personality
Little is known about this creature's nature, although according to Andrias, the being holds wide intelligence and acts as a compilation of previous intellectual minds. Due to this, it desired more intelligence and had a preference for brilliant hosts, even when Andrias attempted to convince the entity to choose someone else. Upon possessing Marcy's body, the Core speaks in a seemingly polite manner as it greets "Why hello there," upon waking up inside Marcy. They also seem to be a bit sardonic, as they taunt Andrias after the Boonchuys and the Plantars defeated his Drone Soldier.

Appearance
The entity's original form was a multi-eyed spherical robot with robotic tentacles and glowing orange eyes with lighter shaded pupils, and in terms of size, it was larger than King Andrias. This form was seemingly connected to Newtopia's castle, given the various wires that were connected to the walls, the presence of its eyes in different locations of the castle, and even one in Andrias' crown for communication.

After possessing Marcy, as Darcy, its consciousness was represented by a dark helmet with four branched gill-like horns, ten eyes and a long wire connected to it. Darcy also wears a new suit, including a long red cape.

Origin
Little is known about its past but long ago, after studying the Moss Man and the Shadowfish, the greatest minds of Amphibia would be combined into an enigmatic entity known as the Core.

Beginning the path of darkness
After Anne, Marcy and the Plantars were successful at their mission at the first temple, King Andrias informs the Core that things are going accordingly as planned, and they will have their revenge.

The Core had been watching Lady Olivia and General Yunan the entire time they were trying to rescue Marcy, and made itself known after Andrias thwarted them. It now desired a host, and it chose Marcy because of her intelligence. While Andrias had begged his master to reconsider someone else, he ultimately gives Marcy to the Core as she is placed in a chair and donned a helmet, allowing The Core to enter her body. The transference, unfortunately, works as The Core, now inside Marcy's body, ominously greets Olivia and Yunan.

Sometime later, after King Andrias failed to defeat Anne and the Plantars and destroys the remote, the Core, currently inhabiting Marcy, appears nearby and mocks him, calling him pathetic, only for him to reassure nothing would stop his massive army, not even Anne and Earth.

Trivia

 * In the credits for "Olivia & Yunan", Haley Tju, Marcy's voice actress, is credited for voicing "Darcy", presumably the alias for the Core-possessed Marcy. This is confirmed during the credits for "Froggy Little Christmas".
 * The number of eyes it had on its original body was inconsistent throughout the appearances it made in the show. In "The First Temple" it had 13 eyes, 11 in the Season 3 intro, and 9 in "Olivia & Yunan".
 * It is hinted in "Olivia & Yunan" that originally, the Core was formed by 8 newts' minds, since the room where it was in had the image of 8 newts with 1 orange eye each connecting to a larger eye in the center, like the Core, which has 8 and a larger one in the center.
 * Darcy's helmet has a total of 10 eyes, implying Marcy's mind was added to the Core's hivemind.
 * In the castle and on the banner of King Andrias are several eye symbols that match the appearance of those on the Core, apparently representing the entity's influence.
 * The Core bares resemblance to the Kree Supreme Intelligence, a supercomputer that consists of the greatest minds of the Kree people from Marvel Comic.