  We explain extremal weight crystals over affine Lie algebras of infinite rank
using combinatorial models: a spinor model due to Kwon, and an infinite rank
analogue of Kashiwara-Nakashima tableaux due to Lecouvey. In particular, we
show that Lecouvey's tableau model is isomorphic to an extremal weight crystal
of level zero. Using these combinatorial models, we explain an algebra
structure of the Grothendieck ring for a category consisting of some extremal
weight crystals.
