All the girls in Izuku's current peerage are part of his harem. They all fell in love with him at some point in their lives, and Izuku also fell in love with them. When Izuku asked them to join his peerage they all gladly accepted.

Everyone in Izuku's peerage was asked if they wanted to join, this is how Izuku and his father work, they will not force someone to join unless there is no other choice, ex: a person they deeply love is severely wounded and turning them into a devil is the only way to save them.

The vampire & Succubus races were extremely happy when they found out their Princes had chosen the Devil king's son as their fiancé. This decision made the three races enter into a powerful alliance. Before Izuku left hell with his purge along with his father and his purge, he and his father erected a powerful barrier around the two races sections of hell, and this barrier can only be broken by one of true royal blood.

While the Neko's Princess didn't choose Izuku as her fiancé, it didn't mean the race was left without protection. Before they left hell the Neko's were asked if they wanted a protective barrier but they refused. So instead Lucifer signed a nonaggression pact with the queen of the Nekos. Not even stray devils, the most insane criminals in all of hell, or any devil dared not attack the Neko race, even eons before the pact was signed. in fear of what would happen to them if the King of hell found out. Let's just say that there's a reason that Izuku has 2 Nekos in his purge. And his father has 6 in his. But unfortunately, as the years progressed the pact faded from the memories of all the races.

Familiar-Zolla-(Dragon Princess.)

Zolla's Human form.

Zolla is deeply in love with Izuku but she hasn't dared to tell him how she feels, she is afraid that he won't love her because of how she looks in her dragon form, and how ruthless she can be in battle when called upon. But unknown to her Izuku loves her back and is well aware of how she feels towards him.

What Izuku remembers about his mother is very little, despite what many people believe his father's wife wasn't Lilith but his best friend who rebelled alongside him Zophiel. Sadly she died when Izuku was very young in a fallen angel ambush when she went to sign a treaty with them. When Lucifer found out he went on a full-on rampage killing 50% of the Fallen angels at the time, and he chased them to the deepest parts of hell where they remain to this day.

Lucifer eventually told Izuku what happened to his mother and Izuku carries a deep hate for the fallen angels

Izuku's Mom.

Izuku has nicknames for everyone in his peerage as well as a nickname for Zolla.

Nicknames:

Lilith-Lili

Kelsea-Kels

Techiya- Tchi

Oraya- Ora

Zephaniah-Zeph

Tzitta-Tiz

Asnat-Nat

Xysiethe- Xys

Suki- Ki

Golda- Gla

Sivana- Sin

Rivkah-Rika

Zolla- Z

Izuku and his father still believe that there are 76 pillars in Hell, they don't know that the houses of Dromos, Squee, Okumura, and Michaelis, betrayed them and set up the houses of Beelzebub, Leviathan, Asmodeus, and Lucifer. Then they used magic to change the memories of the devils and make them believe only 72 pillars existed and that hell was ruled by 4 kings. But unknown to them the true Lucifer made sure to keep the most sensitive history of the devils well protected, and those texts were found by the current 4 kings they live in constant fear that their true rulers will return one day and annihilate their houses for daring to take the throne of the true rulers. Sirzechs is especially nervous because he bears the name of "Lucifer" and doesn't want to begin to think of what will happen to him and his family.

The pillars were granted their power from the true Lucifer when they swore their undying loyalty to him, each of the pillars only received a small amount of the power that Lucifer and Izuku are capable of, Izuku and Lucifer can easily remove the powers from the pillars or a single devil with a snap of their fingers.

Izuku's partner-(Devil Gear.)

Zoiryrth. Zoiryrth is the first-ever dragon god, he challenged Izuku to fight and was beaten to an inch of his life. Right before he died he told Izuku that he would be rewarded for beating him. As a result, Izuku got the first and only Devil gear- True ruler-. His gear can be 100% suppressed so it can be undetected.