  We consider the diphoton resonance at the 13 TeV LHC in a consistent model
with new scalars and vector-like fermions added to the Standard Model (SM),
which can be constructed from orbifold grand unified theories and string
models. The gauge coupling unification can be achieved, neutrino masses can be
generated radiatively, and electroweak vacuum stability problem can be solved.
To explain the diphoton resonance, we study a spin-0 particle, and discuss
various associated final states.
