The document describes a holistic user modeling approach for building personalized user profiles. Data is collected from various sources like social media, smartphones, and fitness trackers. This data is processed and used to populate facets of a user profile like demographics, interests, behaviors, and physical states. The profiles are made available via an API to power personalized services while giving users control over their data and privacy settings. A platform called Myrror was developed to create and manage these holistic user profiles.