Extrude the last vertical row, 3 times, to make the back of the torso.
In Front view, Select the lower row of vertices, and extrude 3 or 4 times
to follow the pelvic roundness. Feel free to change this in order to fit
your model.
My low poly characters counts more or less 1250 vertices. If you need much
less, you must limit the row extusions.
After some extrusions ( two or three) it's better to check, in Top view,
the shape progression, to keep the control of your object.
Select, in Front view, an horizontal row of vertices, the one you want to
control, and press, Shift-H keys, this will hide all the other non selected
vertices, so, now, you can easily adjust the shape, in Top view.
In Side view, Select and Extrude down like in the image.
One way to keep the control of your modelling, is to keep the horinzontal
guidelines more or less aligned.
Again, with Open Gl and trackball action you can work like a sculptor. The
advantage of low-poly modelling is that it's quite easy to control the overall
shape.