The correct term in English is model rigger tho, not modeler. The term rig comes from 3d model skeleton rigging, but in live2d it's the process of making the art move.
For those wondering why the manhole covers are round, it's because they have constant width.
This prevents the covers from falling inside the manhole, which could happen if it was a square, since the sides are smaller than the diagonal, for instance.
Which leads to a fun fact: any shape with...