= Solution
The invariant interaction is
$$
\mathcal L_Y
=-\frac12y\Phi^*\Psi^{\mathsf T}C\Psi+\mathrm{h.c.}
$$
because $\Phi^*$ has charge $-2$ while the fermion pair has charge $+2$. Once $\Phi$ has a vacuum expectation value, it generates a Majorana-type fermion mass and can split the two Majorana components of the original Dirac field.
The operator has dimension
$$
[\Phi^*\Psi\Psi]=1+\frac32+\frac32=4,
$$
so its coupling is classically marginal in four dimensions. By comparison, $\overline\Psi\Psi$ has dimension three and its coefficient is a relevant mass parameter.
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