TITLE: Faithful functors from Rel, the category of sets and relations?
QUESTION [5 upvotes]: Are there examples of faithful functors $F:\mathbf{Rel}\to \mathbf C$, where C is a concrete category over Set? Or can it be proved that such functors don't exists?

REPLY [5 votes]: $\mathrm{Rel}$ 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 $\mathrm{Set} → \mathscr C$.