Kalev ben Yefune comes up in the Five Books of Moses. First he is one of the spies that Moses [Moshe] sent to the Land of Canaan. Later he is brought up in the Book of Joshua  as asking Joshua to give him Hebron as an inheritance (as as promised to him by Moshe Rabainu [Moses]). Later he is mentioned in Chronicles. as the husband of a few wives and girl friends and having children from all. This fact is what the Gra uses as proof that the פילגש girlfriend kind of relationship is permitted.

The place to find this information is in the Laws of Kidushin. [This issue comes up in the Rambam, in the Tur and also by Rav Joseph Karo.]