After defeating a Gargantua, a Dolly Kadmon will appear. They're used to create Hero clan demons.
Devil Summoner: Soul Hackers
In order to create Zeed, Dolly Kadmon must be retrieved from the warehouse that has Dr. Thrill's experiments in it. However one of his Gargantua series demons, Gargan Zero, has frozen the warehouse and must be defeated before it is possible to retrieve Dolly.
If the player turns a Dolly Kadmon into a Hero clan demon before getting another, they can get all of the demons of said clan. However, the PSP remake of Shin Megami Tensei: Devil Summoner wouldn't let the player acquire another if a Hero clan demon is in the party.