Dio Brando/Personality and Relationships

Personality
Dio is consistently manipulative and violently domineering, apparently lacking much conscience or empathy.

In addition to his wit and intelligence, it is suggested that Dio has accumulated considerable knowledge as a result of a passion for reading; introduced doing so as a youth, and several times later in the library of his Egyptian mansion; once more while speaking with Enrico Pucci.

Unless provoked or deemed an obstacle, Dio will normally not attack anything or anybody (although probably more out of regarding them as inconsequential, and so not worthy of attention, than anything akin to kindness). He will first try persuading people to work for him, then fight them, if they oppose.

Araki personally considers Dio to be technically bisexual. He remarked that Dio can and will openly offer himself to either gender using any method, as long as the person suits his present desires. Since he is selfishly interested in their capabilities, however, Araki commented that Dio is the type who is incapable of loving another person wholeheartedly.

Phantom Blood
"I reject my humanity! JoJo!"

- Chapter 11, to Jonathan, before wearing the Stone Mask

Dio is ambitious, sadistic, arrogant and megalomaniacal; seemingly able to do anything to achieve his aims.

He frequently bullied and upstaged Jonathan Joestar in their youth in order to place himself in a better position to replace him and obtain the Joestar family fortune. When he is eventually foiled, Dio protects his ambition all the way through to his adulthood; scheming internally.

Dio openly relinquishes his humanity in favor of becoming a vampire.

His personality could be explained by his poor childhood and the abusive relationship he shared with his father Dario Brando. Speedwagon, however, denied this as a cause for Dio's personality, claiming that Dio was simply born this way, and that poverty had nothing to do with it.

Dio's potential for empathy is emphasized in the anime, when Dio is shown to have killed his own father out of revenge for the abuse against his mother, rather than as a step in his scheme to rise in society.

Stardust Crusaders
"Everyone survives in pursuit of peace of mind. ... Aren't you uneasy, preparing to fight me, knowing you may die? ... If you join me, I will give you peace of mind that will last forever."

- Dio to Polnareff, Chapter 248

Dio expresses various philosophies to Stand users he wishes to recruit, and to his already loyal followers. It seems equally likely that Dio does this to manipulate and persuade as that he believes what he says.

Stone Ocean
In flashbacks placed before the events of Part III, in conversations with Enrico Pucci, Dio shows the potential not to be entirely evil, although he tends to approach it; showing a good side to those who are good to him. In particular, he has shown favorable attitudes towards Enrico Pucci, who also showed loyalty in return.

It is implied that Dio wanted to use his Stand to create a perfect world for himself and a handful of subordinates. He enlisted Pucci as his living "conscience", in case he would go crazy and veer "off track".

Family

 * Dario Brando : Dio hated his father and saw him only as a stepping stone in his path to ascend in society. In both the manga and the anime, Dio's hatred stems from the abuse he dealt his mother with his alcoholism, as well as himself. He emphasizes this by spitting on his grave before leaving for the Joestar mansion.
 * Giorno Giovanna: Dio is not shown to have interacted with Giorno, although Giorno keeps a photo of his father in his wallet though it isn unknown where he got it from or if he knows Dio is his father. Also to note is that while Dio was shown to have sucked the blood out of young women, killing them in the process, he had neglected to kill Giorno's mother for unknown reasons.
 * Ungalo: Similar to Giorno's case Ungalo's Mother was left alive. However,he didn't inherit Jonathan's spirit. Ungalo knows his true father only after his fated encounter with Pucci.
 * Donatello Versus: Similar to Giorno's case Donatello's Mother was left alive. However,he didn't inherit Jonathan's spirit. Donatello knows his true father only after his fated encounter with Pucci.
 * Rykiel: Similar to Giorno's case Rykiel's Mother was left alive. However,he didn't inherit Jonathan's spirit. Rykiel knows his true father only after his fated encounter with Pucci.

Allies

 * Enrico Pucci - Dio's encounter with Pucci is the presumably the most prominent example of humanity displayed by him and was his first proper friendship with another person as he showed compassion by healing Pucci's foot when they first meet. Through the time he spends with Pucci, Dio comes to trust him enough to take control of his plan and at one point, Dio forced Pucci's hand into his head and nearly having his Stand Disc removed to test Pucci's loyalty to him.
 * Vanilla Ice - While the former is fanatically loyal to his cause, even willing to chop off his head to prove his worth, Dio notes that in he often forgets about the doors while he uses his stand Cream to put a hole in the wall.
 * Wang Chan - While at first a provider of his secret oriental poison, after becoming a vampire, Dio used him as a scout and aide once his head is chopped off. Despite his dependence on him, he doesn't take his insults to Jonathan lightly and threatens him for it.

Enemies

 * Jonathan Joestar - He initially viewed his adoptive brother as a stepping stone and pest to be driven to despair and abused in order to gain his fortune. After being beaten once, he viewed him as more of threat, feigning brotherly bounds in public, while secretly poisoning George Joestar in order to get his fortune quicker. He intended to kill Jonathan with the Stone Mask, but upon discovering the hidden power to unlock the vampire powers of man, he decided to use it on himself in order to one. After another failed attempt to kill his adopted brother, he decides to have others face him rather than turn him into a zombie himself due to his ego. After being bested by Jonathan, he quickly changes his mind, only to be thwarted again. In their final confrontation, he admits he respects Jonathan (to the point of scolding Wang Chan for scorning him) and is genuinely shocked when Jonathan dies.
 * Joseph Joestar - He needed Joseph in order to complete his merger with Jonathan's body, secretly manipulating his minions to bring him closer and closer in order to drain his blood.
 * Jotaro Kujo - He viewed Jotaro as much of a threat as Jonathan was, if not more, going to extremes to outwit and make sure of his death. While Jonathan was an opponent Dio respected, Jotaro was an opponent Dio feared.