AbstractJSON.cls

public abstract class JSONUnkownFields {
  public Map<String,Object> jsonFields;
  
  protected abstract void parseKnownFields;
  
  public JSONUnkownFields(String jsonString) {
    jsonFields = (Map<String, Object>)JSON.deserializeUntyped(jsonString);
    parseKnownFields();
  }
  
  protected Object getKnownFieldValue(String key) {
    return jsonFields.remove(key);
  }
}

ConcreteClass.cls

public class UserWrapper extends JSONUNkownFields {
  public String firstName;
  public String lastName;
  
  public UserWrapper(String jsonString) {
    super(jsonString);
  }
  
  protected override void parseKnownFields() {
    firstName = (String)getKnownFieldValue('name');
    lastName = (String)getKnownFieldValue('lastName');
  }
}


//new UserWrapper(response.getBody()) ..