The monoidal unit of $\mathsf{Rel}$ is the functor
\[ \mathbb {1}^{\mathsf{Rel}}\colon \text{pt}\to \mathsf{Rel} \]
picking the set
\[ \mathbb {1}_{\mathsf{Rel}}\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\text{pt} \]
of $\mathsf{Rel}$.
