  Lepton collider setups can probe the neutrino sector in the linear seesaw
mechanism. Small neutrino masses are sourced by a tiny vacuum expectation value
of a leptophilic scalar Higgs doublet and are mediated by Quasi-Dirac heavy
neutrinos. These new particles can all be accessible to colliders. We describe
novel charged Higgs and heavy neutrino production mechanisms that can be
sizeable at $e^{+} e^{-}$ or $e^{-} {\gamma}$ colliders and discuss some of the
associated signatures. These may shed light on the Majorana nature of neutrinos
and the role of lepton number and lepton flavour symmetries.
