At the beginning of the series, Black Star starts off as rather arrogant, loud and immature. Although he fights as an assassin, he prefers to forget the Way of Assassination by shouting and making his presence known, the reason being that he is a massive attention-seeker. He cannot stand having someone else get more attention than himself, and will proceed to beat down anyone more popular than him so he can be popular again. His loudness is often shown in his tendency to often shout when he speaks and his trademark cries of 'Yahoo!' (ひゃっはあ! Hyahhaa! in Japanese).
His arrogance stems from the fact that he seems to exhibit a superiority complex. His delusion of 'starness' as he calls it (スター性 Sutaa Sei) is so great that he constantly gives out autographs that nobody wants, including when he signs Maka's face when she is recovering from a recent battle. According to Stein, Black Star's soul is 'a terribly self-absorbed soul.' His catchphrase is 'I will be the man who will surpass God,' which he is actually trying to do according to Stein. Because of this, he shows tacit jealousy towards Death the Kid. When referring to himself, Black Star uses an honorific (俺様 Ore-sama, literally translating to 'the great me') which shows he holds himself with high regard. He constantly refers to himself as 'BIG' and the other people around him as 'small.' His arrogance later develops into a constant hunger for power, up until the point that people suspect that he may choose the 'Path of the Demon.' His arrogance can also hinder him as he is short-tempered and uncooperative when it comes to team battles, as he always tries to take the lead when the most important thing is to cooperate with each other. He accuses the people around him for being sloppy instead of admitting that he is the one getting frustrated. His confidence stems from him being used to winning all the time, so consequently, losing prominent battles causes a great negative change in him, up until the point he loses his ability to use Soul Menace.
His hubris does not allow him to take any insults on his abilities, such as when he disregards Nygus when she tells him that he is only human. He can be unobservant, and has a short attention-span, as he is shown not to remember their battle plans on the Lost Island mission. He hates being told what to do and thus mostly disobeys the Shibusen rules, often skipping classes and arriving late because of picking fights with people. He likes fighting as he sees it as a chance of proving every one that he is the best. Another thing is that he is incredibly tactless, and can sometimes be disrespectful, even insulting to his friends and even his superiors. He often has an informal way of speech and behaviour, even towards people of a higher rank than him. He loves playing practical jokes and winding people up for the fun of it. He is also a big glutton, having a massive appetite. He seems to like manga, and is a fan of the fictional manga hero, Charisma Justice.
All of his methods of going about solving anything are generally headstrong and dependent on physical strength. From time to time, he will abandon Tsubaki if she tells him to hold back or run away, which ends him up in a worse situation. This might come from people mostly seeing him as a member of the Clan of the Star and judging him for their actions rather than his own. However, he does not hold grudges against Shibusen, the organization that had killed the Clan of the Star, saying that his father and mother had paid for their crimes, which shows a surprisingly mature side to Black Star's personality. He does not like to dwell on the past, and is therefore, quick to forgive people. He tries to forget his heritage, but deep down, he is shown to be insecure about it.
Despite his rather large ego, there are more positive sides to his personality. Although it is hard to tell through his arrogance, he does genuinely care for his friends. He always arrives to his friends' aid if they are in danger, seeing himself as a protector of the people whom he classes as weaker than himself. When Black Star saves Maka from Sid, the first thing he says when Maka thanks him is, 'You're small and I'm BIG. There's no problem about it.' It is shown in team battles, that he views the safety of his friends as a priority. However, he will also take his friends' wishes into account, and will step back and allow them to fight alone if they want to. He also has a determination that knows no bounds. He constantly trains to become stronger, and is dedicated on his goals, putting all of his effort in everything he does (this has its downsides, especially when Black Star develops tunnel-vision on pursuing something that is wrong, being too stubborn to see the wrongness of his aims). He prefers to go his own way, which sometimes involves going against orders. However, despite his determination, he always fails at tests of intellect regardless of the effort he spends in studying. He never forgives the people who hurts his friends, setting off immediately to get revenge in order to defend or restore respect to his friends.
He also has a very positive and upbeat temperament (although he has his short-tempered and cheeky side) often laughing and smiling even in the glummest of situations. His cheerful demeanour can sometimes act as a support to others when they are down. He is also very honourable, not wanting to take any credit that he does not deserve. Tsubaki also mentions that although he tries to hide it, Black Star would never want to stand out for doing something bad. He detests the idea of betrayal or deception, up until the point he is angered when he realizes that the fact that he had killed Mifune is kept a secret from Angela. He also keeps on ploughing forward through adversity, whilst others may decide to run away, showing his bravery, although his bravery may sometimes border on recklessness.
Mifune and Death the Kid are prominent figures in Black Star's life, and they help him mature profoundly. When Black Star is frustrated and indecisive in his pursuit of power, they both help him decide his choice in life. During and after the infiltration of Baba Yaga's Castle, Black Star changes dramatically. He retains most of his positive traits, remaining honourable and sincere both in and out of battle, and still cares for his friends, but most of his negative traits, like his arrogance and his temper, are no longer present. He no longer declares that he is going to surpass God, and he actually starts to admit his faults, saying that some people can do things that even he cannot do. He also develops more respect for his superiors, treating people more like equals. He is more cool-headed and focused in battle. He no longer pursues power in order to stand out and surpass God, but to protect his friends and help maintain order in the world. He decides not to take the 'Path of the Demon' but the 'Path of the Warrior' and he seeks to become a 'Warrior God' (武神 Bushin). He also develops a great sense of responsibility, as he decides to redeem the names of the ones fallen by the Uncanny Sword and to grant Masamune's wish to master the way of the warrior, even if it means causing suffering to some people.
However, he is still slightly immature. He can still be a little too informal and to people, and sometimes exhibits his usual headstrong and proud ways, but it does not go over a manageable level. His childish sense of humour still sometimes embarrasses people, especially Tsubaki.
In the Lust Chapter of the Book of Eibon, Black Star switches gender, and his personality becomes more feminine. He becomes a rather clingy and attention-seeking young girl. He changes back to his male form rather quickly, showing that he is not that lustful, despite his attempts to sneak into the bathroom to peek on a naked Tsubaki (which consequently causes him to start screaming at the sight of Tsubaki, which gives him away).
