  The low energy physics of both graphene and surface states of
three-dimensional topological insulators is described by gapless Dirac fermions
with linear dispersion. In this work, we predict the emergence of a "heavy"
Dirac fermion in a graphene/topological insulator hetero-junction, where the
linear term almost vanishes and the corresponding energy dispersion becomes
highly non-linear. By combining {\it ab initio} calculations and an effective
low-energy model, we show explicitly how strong hybridization between Dirac
fermions in graphene and the surface states of topological insulators can
reduce the Fermi velocity of Dirac fermions. Due to the negligible linear term,
interaction effects will be greatly enhanced and can drive "heavy" Dirac
fermion states into the half quantum Hall state with non-zero Hall conductance.
