  Disordered systems exhibiting exponential localization are mapped to
anisotropic spin chains with localization length being related to the
anisotropy of the spin model. This relates localization phenomenon in fermions
to the rotational symmetry breaking in the critical spin chains. One of the
intriguing consequence is that the statement of Onsager universality in spin
chains implies universality of the localized fermions where the fluctuations in
localized wave functions are universal. We further show that the fluctuations
about localized nonrelativistic fermions describe relativistic fermions. This
provides a new approach to understand the absence of localization in disordered
Dirac fermions. We investigate how disorder affects well known universality of
the spin chains by examining the multifractal exponents. Finally, we examine
the effects of correlations on the localization characteristics of relativistic
fermions.
