  This companion paper to arXiv:2109.00065 explores the interpretation of
$AdS_3/CFT_2$ duality for $k=(R_{AdS}/\lstr)^2<1$ in terms of "non-critical
little string theory". We review the underlying fivebrane structure of 1/2-BPS
backgrounds for string theory in both the critical dimension and for
non-critical models. D-branes bound to the fivebranes in these backgrounds are
associated to nonabelian little strings, and flesh out a picture of the
correspondence transition at $k=1$ wherein the nonabelian little string
excitations lose their Hagedorn entropy, allowing the Hagedorn entropy of
fundamental strings to dominate the asymptotic density of states for $k<1$
instead of BTZ black holes. We then discuss the application of these models as
approximations to and probes of black hole dynamics and thermodynamics, as well
as associated information puzzles. In particular, because the correspondence
transition reveals the interior of an evaporating black hole, it serves as a
sensitive probe of any proposal regarding the composition of that interior.
