well, you can use spriter, or you can use blender,
with blender you can "project" the 2d sprites as textures for a polygon rig, a 3d artist can do a 2.5d rig for musculature so the that can stay consistent.
example this animation is made by projecting a 2d image to some polys
https://www.youtube.com/watch?v=s_lunhrD-Dg
https://www.youtube.com/watch?v=9LgGrwkUCGE
https://www.youtube.com/watch?v=fMO_dMZrUIY
https://www.youtube.com/watch?v=cgWbWGs7WfA
this guy uses crazy rigs, but for KOF characters, you don need so much.
the process overall would be a combination of the videos above & this:
https://blog.gskinner.com/archives/2018/02/rigging-animated-2d-sprites-part-1.html
the advantage is that once this work is done, you can use the bones to make walking animations for lots of characters.