Station Manager

In order to unlock this customer, you will need to have unlocked 70 researchers and then research the area "Candy Investment" which will show up once you have met that requirement. It's best to focus on 1 town to unlock the 70 researchers since customer level are not shared across towns (unlocked researchers carry over however). Under the Info tab, you can view the Customer List, which will show you at what level each customer will become a researcher. Most are levels 5,10, or 15 and not all customers can become researchers. Focus on leveling up your town(s) and unlocking trains and facilities and you will naturally unlock enough customers.