The Echidna is a being and able to .
For her attack telegraphs, that are supported by arms, hands, or hair would work best.
The Swipe attack can start on the left or right side, which needs to be shown in an animation somehow. If that's with any limbs or anything else does not matter, as long as it is clearly visible and readable.
She is a boss, moving only slightly in around her point of origin but without changing the hitbox too much.
Her body is submerged in the River Styx but if that's from down her hips, chest, or any other point does not matter.
She can be rather big and to fit the ancient feel of the Gods and her role as the Hydra's and Cerberus' mother, or closer to as she will be much longer than him anyway. Her size needs to fit to the Snake's body diameter during the Snake Leaps so those requirements need to be met somehow.
the Gods being displayed as gigantic figures
most Gods' sizes are undefined as they don't have comparable sprites in the game
God sprites in-game have varying sizes as can be seen on Hades and Nyx
Monsters have varying sizes, Asterius, the Hydra, the Gorgons, and the Cerberus have vast differences
The Echidna's Snake tail end in a snake head rather than a tail tip.
It is clearly its own actor, but still part of the Echidna's body and mind, so a design that but brings them is best.
The Snake uses a lot of strike attacks so it need to and do so quickly.
It uses both ground strike and river dive attacks.
The river dives, it executes with no problem.
The ground strikes it when it hits the ground too hard so the head is not fully armor plated to an extend where it would prevent the Snake from being stunned.
During the Snake Leap attacks, the Snake of the body.
It dives through pools or rivers that might be as narrow as its body, meaning the Echidna needs to assume the at that point.
Additionally, having the Echidna simply flail around at the end would make it look really silly so another solution needs to be found. The Echidna as the body forms walls around the level that need to disappear at one point while the body is dragged after the head.
the Echidna is dragged along the Snake somehow
The Echidna and the Snake both need a character portrait.
It can be shared, although the UI and optimally the focus needs to shift between them as they both get text lines and voice acting / sound.