If your Custom Object is connected to another object through a lookup relationship field, you can add that object's fields to the Custom Object report.
To add the lookup relationship fields
- Go to Reports > Other Reports.
- Select a Custom Object report or create a new report using the Custom Object.
- Under Report Fields, find the lookup relationship field you want to add. Alternatively, you can use the search bar under Report Fields if you know the name of the field. Drag the field from the list onto the report.
- When you're finished editing, click Save.