You can decorate the entity class and serialize it directly but this is not advisable. This can cause problems especially with navigation properties. In general we always advice to use dedicated DTO objects that define explicitly the contract object that will be serialized.
If you decide to serialize the entity object directly I would advice to create a detach copy of the object and serialize this new object. The reason is that in Telerik Data Access - navigation properties are lazy loaded. This means that out of the box when the serializer calls a navigation (reference or collection) property it will be loaded from the database and then serialized. The result is that if you ask for a single object you may load substantial portion of your data if there are navigation properties in the target entity type. To overcome this behavior I suggest that you use CreateDetachedCopy method to disable lazy loading during serialization and control what portions of your object graph is sent to the client.
Also for an example please check
Dimitar Tachev answer here
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