"The only child of the Demon King and the most powerful being in the Devildom. He is also president of the RAD student council."
Diavolo is the ruler of all demons, and all who reside in the Devildom know of his name. Although his father, The Demon King, is still alive, he has no interest in the affairs of the three worlds. He allows Diavolo full reign over the Devildom, choosing instead to sleep at the bottom of the Devildom.
As a result, it is taken as truth that Diavolo will be crowned as the Future King of the Devildom. As such, Diavolo has begun strengthening the Devildom's relationship with both the Human World and the Celestial Realm through the exchange program, as he believes that the three worlds should live harmoniously with each other.
Diavolo has layered, red hair with his bangs parted to the left side, tan/dark skin (depending on his form), and pale golden eyes. Like the brothers, he too has painted nails - black, in his case.
Despite being one of the most powerful demons in the Devildom as well as the Future King, Diavolo takes on a surprisingly fair and compassionate, if not joyous, attitude at almost all times. However, that is not to say he is a soft or lenient ruler, as even without the threat of retribution from Lucifer, all demons appear respectfully intimidated by him.
Diavolo proves to have much more wisdom than his cheerful demeanor suggests. He shows moments of solemn clarity in which he correctly predicts events that will occur in the future either due to his subtle encouragement or his intentional lack of intervention. He has the ability to tell if someone is telling the truth and has made a rule for himself to never tell a lie.
The goals that Diavolo sets for himself, his fellow demons, and the three worlds at large, appear to be pursued with the utmost best intentions. As evidenced by the fact that so far he has only ever done what he has done with the desire to improve the lives of those he rules over, even if the process is painful.
- His favorite treat is hellfire mushroom rolled cigar cookies, as mentioned in the Devilgram for Lucifer's Melancholy.
- He also states that he likes custard in the Devilgram Hippity-Hoppity.
- In one of Lucifer's dialogues, he mentions that Diavolo likes bathing in the light of the moon.
- He hates pickles, as revealed in the chat Diavolo's Natural Enemy.
- He also dislikes bell peppers and garlic anchovy dip according to the chat How to Feed a Picky Eater.
- In the Devilgram The Escorts, Diavolo expresses dislike for the Great Witch Maddi, who wanted to marry him.
As he tells MC in Lesson 56, Diavolo grew up very sheltered and isolated. His mother died after giving birth to him, and his father was always very strict with him. He never had the chance to speak to anyone outside the castle until he met Barbatos. After meeting him, Barbatos began to tell Diavolo stories about the outside world, which he liked so much that he asked Barbatos to stay with him. Although this was always refused by Barbatos, Diavolo never gave up. Eventually, he convinced Barbatos to stay by telling him that he would not become the Future King of the Devildom otherwise.
Little is known about Diavolo's past, but his goal has always been to build a better relationship between the three realms. He had met Lucifer when he was an angel in the past and was quite courteous towards him, despite the high ranking angel's contempt for him.
After the Great Celestial War, when the brothers fell from grace, he and Barbatos appeared before Lucifer and a dying Lilith in an area which would later be RAD's colosseum. He offered to help Lucifer's sister if he would pledge his loyalty to him. Then Diavolo had Lilith's soul reincarnated as a human.
Diavolo is the first character to introduce himself to MC when they appear at the student council hall.
Diavolo places great trust in Lucifer, considering him his right hand man and also his closest friend. The two of them often spend time together, drinking tea or discussing important matters. Diavolo often drags Lucifer along with his plans, and seems to enjoy teasing him on occasions, like using the chat stickers Lucifer told everyone to stop sending as he finds them distasteful. He worries for Lucifer a lot, going as far as keeping secrets from Lucifer in order to protect his feelings, even though his decision is disapproved by both Barbatos and Simeon. Once the truth is revealed, it has a negative affect on their friendship, as Lucifer puts some distance between them. Diavolo tries to patch it up by giving gifts to Lucifer and even asking MC for advice on what he should do to earn his friend's trust back.
Barbatos mentions in one of his home screen dialogues that Lucifer is like family to Lord Diavolo. In the chat Hello (Diavolo), he explains sending MC the wrong sticker by saying Lucifer is the only demon who sends him messages.
There is a folder on his computer named after Lucifer, as stated in the event Welcome to the Bunny Show.
- His name is Italian for "devil".
- In an earlier draft of the game, Diavolo was a prominent figure in the human world as a CEO of the Richter Hotel Group.
- According to a psychological test that Asmodeus taught Diavolo — who brings it up in the chat Behavior in Love — he has a want to "constantly observe the target of his affections."