The Pajama Cardinalfish is named because of the polka dotted tail section. When small, these fish are known to shoal together and add some unusual dynamic to the tank. While it only takes a male and a female cardinal to produce fry, multiple females will be happy with a single male companion. Cardinalfish are mouth brooders, meaning that the male will carry the fry in his mouth for about two weeks until they are ready to release, all that while he will forego any meals. His jaw area will be more pronounced, compared to the female counterparts. As these fish mature, their coloration dues fade somewhat.
Cardinalfish are nocturnal, quick to devour a meal in near total darkness - including their own offspring unfortunately. If you have clownfish or others breeding in your reef, Pajama (and Banggai) Cardinalfish will enjoy a live snack as those release into the water column.
Here's a picture of a male carrying some eggs. Usually the daddy keeps his mouth tightly closed, but you may catch him rolling them around, providing a glimpse: