  We pursue a possibility that a pseudo-Nambu-Goldstone boson is lurking around
or below the intermediate scale. To this end we consider an anomaly-free global
flavor symmetry, and construct models where the pseudo-Nambu-Goldstone boson is
coupled preferentially to leptons. The experimental and astrophysical bounds
derived from couplings to photons and nucleons are significantly relaxed. If
sufficiently light, the pseudo-Nambu-Goldstone boson contributes to dark
matter, and interestingly, it generally decays into photons through couplings
arising from threshold corrections. We show that the recent hint for the X-ray
line at about $3.5$ keV can be explained by the decay of such
pseudo-Nambu-Goldstone boson of mass about $7$ keV with the decay constant of
order $10^{10}$ GeV, if the electron is charged under the flavor symmetry.
