  We investigate the one-dimensional defect SCFT defined on the $1/2$ BPS
Wilson line/loop in ABJ(M) theory. We show that the supermatrix structure of
the defect imposes a covariant supermatrix representation of the supercharges.
Exploiting this covariant formulation, we prove the existence of a long
multiplet whose highest weight state is a constant supermatrix operator. At
weak coupling, we study this operator in perturbation theory and confirm that
it acquires a non-trivial anomalous dimension. At strong coupling, we
conjecture that this operator is dual to the lowest bound state of fluctuations
of the fundamental open string in AdS$_4\times \mathbb{CP}_3$ around the
classical $1/2$ BPS solution. Quite unexpectedly, this operator also arises in
the cohomological equivalence between bosonic and fermionic Wilson loops. We
also discuss some regularization subtleties arising in perturbative
calculations on the infinite Wilson line.
