Alucard's most complex interaction is with Clare Nightshade, whom he turned into a vampire after she was mortally wounded by her father's murder (who has also a vampire). Tho he almost always introduces and refers to her as "redhead", something which she finds annoying, it is obvious from his words and actions that he cares for her very much, and it is often hinted that he has romantic feelings for her. Sometimes, after a danger filled event or one of their talks on the nature of humanity, Alucard displays obvious acts of affection, hugging her or patting her head, or even referring to her as "his Claire". When this happens, Claire starts acting shy and nervous, hinting that the feelings are reciprocal. Also, when someone attacks Claire, Alucard will brutally attack that person, telling them not to touch "his Claire". This once again shows how much he cares for her.
Neo is the only person who Alucard has ever considered offering his succesion to. He trully thinks Neo has the power and the mind to one day replace him, as both King and CEO of Red Soul Clinics. Thus, traning Neo to unlock and control his powers is one of his top prioritys. Basivally, Neo could be considered the "Robin" to Alucard's "Batman".
With Neo, Alucard is both extremelly firm, but olso vary caring. He trust Noe above all else, tellin him secrets no one else knows. Neo is also the only person who can use any of Alucard's things without him minding. He has evren stated: "It will all belong to him eventually. Best he get used with it." A testament of Alucard's trust for the boy is when he learned that he liked his "daughter", Ruby. He actually encouraged him to confess his feelings, going as fas as to ask if they had "done it", much to Neo's embarasment.
In comparison, when learning that Ruby liked Peter, he warnwd the werefowlf not to break her heart, or he whould break him.
It is also obvious that, if Alucard whould have to chose between himself and Neo, he whould sacrifice himself for the boy without hessitation.
The two first met it was when Arthur was twelve and Alucard was breaking into their house to fight Gabriel. After a rather one sided 'fight' Arthur and his sister were more or less made to give up, not being able to hurt Alucard due to their inexperience. During this encounter Arthur attempted to kill Alucard multiple times, but Alucard simply dodged and left after he finished his conversation with Vivien.
After his reformation he met Alucard a second time, under his alias 'The Butcher's Herald' when Alucard was investigating death threats against local Werebear populations in Denmark. After a short fight, Arthur was unmasked by Alucard with Arthur revealing his reformation, although remaining silent on his reasons why. Alucard offered to help his defeat his father, which Arthur took as an insult and stabbed before fleeing. They would meet a few times again over the next few years, up until Arthur and Vivien were forced underground by the Order, and during this time adopted a sort of strange friendship.
Alucard has had a profound impact on Arthur's view on the supernatural, and Arthur has provided Alucard some insight into his rivals past. Interestingly Arthur treats Alucard with even more respect then he gives other people, calling him sir, due to him realizing just how old Alucard is and all the good he has done, as well as having sympathy for the pain he sense Alucard feels. In addition Arthur has expressed a wish to spar with Alucard which the vampire frequently brushes off. He also once gave Alucard blood, due to him being immune.
Vivien knows Alucard from a young age and is thankful towards him for proving that her early suspicons of being lied to were in fact correct, and for helping her and Arthur during their time on the run from the Order. She still notes that she finds Vampires creepy, and is still a little upset that her parents were killed by them, but also is stright up with the fact that Alucard proved to her the actions of a few do not reflect the majority.
Alucard himself notes a bit of sadness after finding out she has become a werewolf, as he states if her father were still alive he would have killed her: to which Vivien casualy tells Alucard, "balls to my dad."
A long time 'rival' of Alucard, they have fought so many times both of them have lost count. While Gabriel despises Vampires in all their forms, he admits to having a tiny amount of respect for Alucard due to his ability to being the only individual to consistently beat him. Their win loss record is spotty, but Gabriel begrudgingly admits he is confident that Alucard is winning by four fights, even though he insists three of them were because he used his 'dragon form.'
Despite this respect Gabriel maintains a 'kill on sight' policy with Alucard, identifying him as the Red Dragon from the 1812 "Grim day massacre" in England. He is also personally angry at him for puncturing one of his lungs, forcing him in combat to wear a breathing apparatus in his helmet. Although takes smug satisfaction in being probably the only man to ever cut off Alucard's arm in a one on one duel, although is very angry that he could simply reattach it.
Still on his death bed, Gabriel went to Alucard and bore his soul. With the Order's betrayal and the acceptance of his Children's future he visited the one person he really knew. There he spoke to the Vampire for the first and only time in a civil fashion. And although they disagreed on a lot of things, Alucard offered to turn him into a vampire, which he rejected and Gabriel made him promise to look after his children. which Alucard accepted. Alucard was the last person Gabriel spoke to before he died, and in death Alucard has nothing but respect for Gabriel as he had the will to admit to the Vampire that many of the things he did were wrong.