If you've just taken it out of your inventory, it doesn't seem to have bladder, hygiene, or energy needs. I have seen an imaginary friend eat - they eat donuts and rainbow cookies. When the sim it belongs to goes to bed, the imaginary friend will run to the foot of the sim's bed and turn back into a doll - it will "come alive" again when the sim wakes up if you don't put it back in inventory.
If you turn it "real", it joins your household and is just like a regular sim in most ways.