  Let $R$ be a commutative ring with an identity, and $S$ a multiplicative
subset of $R$. In this paper, we introduce the notion of $S$-injective modules
as a weak version of injective modules. Among other results, we provide an
$S$-version of the Baer's characterisation of injective modules. We also give
an $S$-version of the Lambek's characterization of flat modules: an $R$-module
$M$ is $S$-flat if and only if its character,
$Hom_{\mathbb{Z}}(M,\mathbb{Q}/\mathbb{Z})$, is an $S$-injective $R$-module. As
applications, we establish, under certain conditions, counterparts of Cheatham
and Stone's characterizations for $S$-Noetherian rings using the notion of
character modules.
