Understand what capabilities are provided by Pega layers (like Pega Foundation for Financial Services), such as - but not only - the data model provided. The 'focus' needs to be delivering a business outcome for your client, but where pre-developed and pre-tested capabilities from a Pega layer can accelerate that, it typically makes sense to leverage those rather than implementing a new class model from scratch.
Undertake an exercise during the Prepare Stage to identify alignments between the client's data model and Pega's. Implement Classes in Client Rulesets that inherit from - or are composed of - the aligning Pega Classes. That which is genuinely Client-specific, and needs to be retained, can be implemented as an increment to the Class in the Client Ruleset.
Avoid letting the client's incumbent physical data model (how their business objects are persisted in a database, or transmitted via an integration) influence your Data class design. Rather, ensure your Data classes mirror how the business data is used by your Pega application (e.g. a Customer has a List of Addresses).
Defer then to Data Pages which solve the problem of transforming between business objects and data stored or transmitted. For example, when Customer Data is stored across a CUSTOMER table and a CUSTOMER_ADDRESS table.