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; he is introduced doing so as a youth, and several times later in the library of his Egyptian mansion; and once more while speaking with Enrico Pucci.

Dio will normally not attack anything or anybody unless provoked (although probably more out of regarding them as inconsequential, and so not worthy of attention, than anything akin to kindness). Even when trying to remove an obstacle, he will first try persuading people to work for him, and only 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 both the manga and 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. However, this time, he seems far more calmer and collected and uses his intelligence to bide his time during his recuperation and completion of his head on Jonathan Joestar's body instead of rushing headfirst into things; this patience and greater focus on his intelligence is shown when he checks every possible way to see if Jotaro had truly been killed during their fight. When he takes Joseph's blood to heal himself from Jotaro's surprise attack, the blood not only healed him, but it also completed the fusion of his head to Jonathan's body and brought him back to full strength, causing him to behave and act like he was in Phantom Blood.

Stone Ocean
"...Beyond the powers of my Stand is where you need to go in order to find Heaven. What you need is a trustworthy friend ... Will I, DIO, be able to meet someone like this one day?"

- Dio's Diary

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 hesitantly, only 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.
 * Dio's mother: Though she isn't seen interacting with him, it's very clear that Dio loved his mother deeply, and the abuse she was dealt by his father was one of the reasons Dio slowly and painfully killed Dario.
 * Giorno Giovanna: Dio is not shown to have interacted with Giorno, although Giorno keeps a photo of his father in his wallet. However, it is 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 because he felt she was abusive, as revealed in Over Heaven.
 * Ungalo, Donatello Versus, and Rykiel: Similar to Giorno's case, the mothers of all three men were left alive. However, none of them inherited Jonathan's spirit. All three know their true father only after their 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 met. 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 had 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 Vanilla Ice 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 a 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 become 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.
 * William Anthonio Zeppeli: Due to Dio having used the Stone Mask and becoming a Vampire, Zeppeli immediately viewed Dio as an enemy once he learnt about the incident at the Joestar Mansion and due to his own father using the same mask to become a Vampire himself.
 * Robert Edward O. Speedwagon: After hearing from Jonathan what Dio was doing to George Joestar I, Speedwagon decided to help with finding the man that sold Dio the poison. After everything that happened at the Joestar Mansion, he went along with Jonathan and Zeppeli to defeat Dio, knowing that he was too dangerous now to be left running around.
 * Dire, Straizo, and Tonpetty: Being Hamon Warriors, the three knew the dangers of the Stone Mask and viewed Dio as an enemy because of him being a Vampire. Out of the three of them, Dire was the one who really wanted to defeat Dio on his own, due to Dire having been a friend of Zeppeli's for many years and Dio having a role in his death. At the time, Straizo knew that defeating Dio was important; however, years later, he would admit to Speedwagon that he was envious of Dio due to the fact he would never age while Straizo would. This envy became so much that he would later use the Stone Mask on himself just so he could become young again.
 * 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. Joseph had heard stories of Dio from Erina when he was small and knew of the danger he would bring. His hatred for Dio intensified, however, when he learnt that Dio had stolen the body of his grandfather, Jonathan Joestar, to use as his own. This hatred got to the breaking point when Dio's very existence threatened the life of Joseph's daughter Holy Kujo.
 * Noriaki Kakyoin: Having met Dio when on holiday in Egypt with his family and then put under his control with a Flesh Bud, Kakyoin would do anything Dio wanted. But upon having the bud removed by Jotaro and seeing the harm Dio was causing on Holy Kujo, Kakyoin decided to go with Joseph, Jotaro, and Avdol to Egypt to defeat Dio.
 * Jean Pierre Polnareff: Like Kakyoin, he too met Dio, who then placed him under his control. After having the Flesh Bud removed by Jotaro, Polnareff decided to join the team, though at first for his own goals, as Polnareff theorised that Dio would know where his sister's killer would be. After the deaths of both Avdol and Iggy, Polnareff vowed to defeat Dio no matter what.
 * Mohammed Avdol: Unlike Kakyoin and Polnareff, Avdol knew who Dio was immediately upon meeting him due to having heard about him from Joseph; as such, he knew he was an enemy right off the bat and managed to escape from him before he could be put under his control. With Dio threatening Holy's life due to him activating all the Stands in the Joestar bloodline, Avdol, along with Jotaro, Joseph, and Kakyoin decided to journey to Egypt to defeat Dio.
 * 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. Jotaro, on the other hand, wanted to defeat Dio as quickly as he could due to his mother dying from her Stand, a Stand caused by Dio activating all the Stands in the Joestar bloodline at the same time. After Dio killed Kakyoin and apparently killed Joseph, Jotaro was hellbent on killing Dio.