Let $(R,+,\cdot)$ be a finite ring. Obviously, $(R,+)$ is an abelian group; however the unit (multiplicative) group need not be abelian.

**Definition** Given the dual group $\widehat{R}$ of $(R,+)$, *character* $\chi\in\widehat{R}$ is said to be *primitive with respect to a collection $\mathcal{C}$ of subgroups* of $(R,+)$, if $\chi$ does not descend to a character on any quotient group $R/S$ for all $S\in \mathcal{C}$. To descend here is canonical in the sense that $\chi$ is invariant on the elements of any coset of $R/S$.


My question is the following problem:

> Are there any generic non-trivial conditions on $\mathcal{C}$ such that
> $$\#\{\text{primitive* characters in $\widehat{R}$ w.r.t $\mathcal{C}$}\}=\#R^\times,$$
i.e. when does the number of primitive*
> characters of the abelian group $(R,+)$ w.r.t $\mathcal{C}$ equal the number of units in
> $R$? 

**NOTES**\
The archetypal example is the ring $\mathbb{Z}/n\mathbb{Z}=(\{0,1,2,\dotsc,n-1\},+,\cdot)$ of integers modulo $n$ where the answer is positive when one considers all proper quotients.

**(Added later)** Qiaochu Yuan's answer shows that  $\mathbb{Z}/n\mathbb{Z}$ is indeed the only example when one considers all proper subquotients. My question is whether a well-defined non-trivial collection of, rather than all, quotients leads to an affirmation of the equality above.