from the three of them, i always thought the teacher is way too obvious to be rum, and from the chapters they went to the farm, it showed that she probably knew about the murder of haneda. so, only the other two become suspicious. for the police chief, i thought it's kinda way too overpowered since he could have any data for any cases, but still possible. and for the sushi chef, i thought he's somehow easily amazed by kogoro and also he kinda stepped some boundaries before(?). but finally, we know who is it.