I am working for a non profit organisation and Our Project needs an android app (to develop a brief intervention that uses mobile text/voice messaging to address problematic drinking among adolescents.) This app should woks as a bidirectional and better if it collaborate with watsapp. so that we will get responses quickly via watsapp and automatically our apps should tell questions base on his/her answer. Also it should store conversation data into the database.
You can start from the friendly chat app based on Firebase. Take a look here https://codelabs.developers.google.com/codelabs/firebase-android/#0 . For the "auto reply" functionality you will have to write your own cloud function. Read some docs about that