Although it initially looks like the Boktai series is using a dimetric projection (135, 135 and 90 degree angles between the axes), I think it's actually Oblique. The proportions of the axes' foreshortening seem all over the place, giving off that odd, stretched-out look.
I think the artists' approach was to first draw an object as it would appear looking directly over it, maintaining its width and length (x and z axes). Next, the view was rotated 45 degrees to give it that "isometric" feel. After that, the height lines were added as necessary without really maintaining proportions. The end result of this is Oblique projection, which was used in a very similar fashion in
Ultima Online.