  We report {\it ab initio} calculations of the structural, electronic and
magnetic properties of a graphene monolayer substitutionally doped with Co
(Co$_{sub}$) atoms. We focus in Co because among traditional ferromagnetic
elements (Fe, Co and Ni), only Co$_{sub}$ atoms induce spin-polarization in
graphene. Our results show the complex magnetism of Co substitutional impurites
in graphene, which is mapped into simple models such as the $\pi$-vacancy and
Heisenberg model. The links established in our work can be used to bring into
contact the engineering of nanostructures with the results of $\pi$-models in
defective graphene. In principle, the structures considered here can be
fabricated using electron irradiation or Ar$^+$ ion bombardment to create
defects and depositing Co at the same time.
