Regulatory subunit of a potassium efflux system that confers protection against electrophiles. Required for full activity of KefC. Shows redox enzymatic activity, but this enzymatic activity is not required for activation of KefC. Can use a wide range of substrates, including electrophilic quinones, and its function could be to reduce the redox toxicity of electrophilic quinones in parallel with acting as triggers for the KefC efflux system. a quinone + H(+) + NADH = a quinol + NAD(+) a quinone + H(+) + NADPH = a quinol + NADP(+) Homodimer. Interacts with KefC, forming a heterotetramer with 2:2 stoichiometry. Belongs to the NAD(P)H dehydrogenase (quinone) family. KefF subfamily.