Helix was a Kaiju during the second Kaiju war.
Helix is probably one of the most unique Kaiju ever to exist, since it wasn't just one, it was actually many thousands of small Kaiju at once. They manage to do this by replicating what humans call The Drift, each sharing each others minds, and never breaking away from the drift. They all act as one Kaiju, but can each make their own decisions if it is not overridden by the others. Each Kiju is about a meter long and a foot tall, and look like short centepides with large mouths and spines.
When all of them assemble, they can form into many shapes. The shape encountered by Nyhon No Ryu was vaguely human shaped, with a huge mouth in the center of its stomach.
History can be read right here- Helix history, third section.