$\mathrm{Rel}$ is itself concretizable by the functor $U : \mathrm{Rel} → \mathrm{Set}$ defined on objects by $UA = 2^A$, and on morphisms as a generalized image. Given a relation $R ⊆ A × B$ and a subset $X ⊆ A$, set $UR(X) = \\{b ∈ B : \exists x ∈ X, \; xRb\\}$.
To get other examples of faithful functors $\mathrm{Rel} → \mathscr C$, you can now compose $U$ with faithful functors $\mathrm{Set} → \mathscr C$.