First, I am an Amazon user and do not use Google. However, I have provided this information to other players who have completed the survey's and those players are able to obtain DQ IAPs through using google credits they earn from the Google Opinion Rewards app. It might take several surveys to obtain enough credits to obtain a character slot but it works, where applicable. In order for this to work, you need to have your account and dungeon quest through The Google Play Store. I know it does work in the US and UK but I'm not sure about anywhere else in the world. Either way, it is a way to obtain character slots without spending money.
Google Opinion Rewards Description: Answer quick surveys and earn Google Play credits with Google Opinion Rewards, an app created by Google Consumer Surveys.
Getting started is easy. Download the app and answer basic questions about yourself. We'll then send you surveys around once a week, although it may be more or less frequent. You'll get a notification on your phone when a short and relevant survey is ready for you, and can receive up to $1.00 in Play credits for completing it. Questions can range from, "Which logo is best?" and "Which promotion is most compelling?" to "When do you plan on traveling next?"