  New light singlet scalars with flavor-specific couplings represent a
phenomenologically distinctive and flavor-safe alternative to the well-studied
possibility of Higgs-portal scalars. However, in contrast to the Higgs portal,
flavor-specific couplings require an ultraviolet completion involving new heavy
states charged under the Standard Model gauge symmetries, leading to a host of
additional novel phenomena. Focusing for concreteness on a scenario with up
quark-specific couplings, we investigate two simple renormalizable completions,
one with an additional vector-like quark and another featuring an extra scalar
doublet. We consider the implications of naturalness, flavor- and CP-violation,
electroweak precision observables, and direct searches for the new states at
the LHC. These bounds, while being model-dependent, are shown to probe
interesting regions in the parameter space of the scalar mass and its
low-energy effective coupling, complementing the essential phenomenology of the
low-energy effective theory at a variety of low and medium energy experiments.
