The five familiars can be obtained by collecting card relics from locations in the normal castle. Whilst it isn't necessary to have the bat form ability, it will make things much simpler.
Long Library. Use bat form to get to fly to the ceiling, then to the door in the top left.
In the Slogra and Gaibon boss room. Use bat form to fly to the ceiling, then to the door in the top right.
Olrox's Quarter's. Find the fountain, then go through the lower left door. This room has a weak ceiling on the left side which can be broken and jumped through.
Abandoned Mine. Find the unusable switch. Travel one room down, then keep heading left.
Castle Keep, at the highest area in the normal castle. While outside, use bat form to fly as high as possible, then go through the window on the right. It is in the bottom right corner of the room.
The video below (not mine, credit to Smeer516) shows a visual guide to the locations (lincluding maps).