Black Star is one of the main characters of Soul Eater. He is student as Shibusen, and uses the demonic ninja weapon Tsubaki.
Sid cannot leave the young Black Star to die
Black star was the son of the legendary ninja clan known as Hoshizoku, which translates to "Clan of the Star." They were well known for being a clan that was feared for being brutal and for ninjas for hire, with his father White Star being the most feared and ruthless of them all. The clan was so dangerous that it was exterminated by Shibusen, with Black Star, who was only an infant at the time, being the only survivor. One of the members of Shibusen's task force, Sid Barett, found the infant Black Star and couldn't leave him there all by himself. So he helped to raise the child and has filled in almost as a father figure for him as Black Star attended the Shibusen school and became a high level meister along with his partner Tsubaki.
Years later Black Star is introduced into the series when he and Tsubaki try to track down more Kishin souls to eat so that Tsubaki can try to become a Death Scythe. Even though both of them were very talented, they had yet to devour any souls and in order to become a Death Scythe Tsubaki had to devour ninety nine normal Kishin egg souls and one witch's soul. However Black Star continues to screw this up because he cannot abide by the Assassin's Code of stealth, he demands that he announces his presence and greatness whenever he attacks someone, giving them time to run away. However he is told that if he defeats and devours the soul of a truly powerful opponent then that will be like eating 99 souls at once. So he hunts down Mifune, a legendary swordsman, and thanks to an illusion he is able to defeat him. However when he finds out that Mifune is guarding a young witch by the name of Angela, he cannot bring himself to take him away from her.
Frank N. Stein and The New Student
Black Star and Maka set a trap for Sid
At school Black Star learns that his mentor Sid has died and been turned into a Zombie. Black Star teams up with Maka and Soul to hunt down Sid and find out who did this to him. After battling Sid Black Star is able to trap him with some chains and then uses some rather pervy methods to get him to spill his beans on who made him a zombie. They find out that it was Frank Stein, the mad scientist of Shibusen. They hunt him down and quickly discover they are not strong enough to defeat him, it even looks like Stein kills Black Star at one point. However they learn that Sid became a zombie so he could get stronger and Frank Stein was hired to just tests the kid's powers and abilities as an extra lesson. With that complete they returned to the school with Stein as their new teacher.
Black Star and Soul tell Kid to bring it
However Stein is not the only new addition to the school, Death the Kid comes to the school as a new student and immediately starts rubbing Black Star the wrong way. He teams up with Soul to fight Kid, however they are no match for him. They try and team up, with Black Star using Soul as his weapon, however their souls aren't in unison together so he can barely move Soul at all. Kid defeats them both by using his own Soul Resonance attack on them. After the battle though Kid faints because he got a slight bit of his hair cut during the battle, so Black Star says he actually won the match.
Excalibur makes Death and Black Star look like "Fools!"
Even though Kid had beaten him in battle, he and Black Star got along well at school. One day while being punished for his goofing off, Black Star was being forced to clean the library, it was then that Kid came in and told him that he was researching a legendary sword by the name of Excalibur. The two of them decided to set out after this sword and both encountered the weapon of legends. However neither of them could stand him for more than a few minutes, being driven insane by his constant babbling and pompous behavior. They stuck the sword back into its shrine and left, although even though they seemed to be leaving empty handed, they had in fact gained a new level of respect for each other.
Black Star faces off against a possessed villager to free him of Masamune's control
The Demon Weapon Masamune, the older brother of Tsubaki was on his way to becoming a Demon God, traveling the countryside killing innocent victimes, so Black Star and Tsubaki took it among themselves to be the ones to stop him. When Black Star arrived in the village Masamune had been spotted in, the villagers started referring to him as a member of the Star Clan, and offered him money to leave, saying that that's what his clan did, they did anything for money. Black Star told Tsubaki that he knew all about his clan's past and about how Shibusen had slain them all, but he held no grudge against them. When Masamune arrived he took over the body of one of the villagers in the town, and Black Star had to defeat him before Tsubaki could travel into the cursed sword and face her brother one on one. While in the sword Black Star watched from the outside, while the villagers all attacked him. However no matter how hard they beat him he didn't budge, he remembered how when he arrived at Shibusen Tsubaki was the only one who listened to him go on about how he would one day be a great star, so now it was his time to watch and wait for her to finish. Once she appeared out of the sword, victorious over her brother, Black Star and Tsubaki fled the town, being chased out by angry villagers.
Black Star using Tsubaki's new powers
Tsubaki gained a new mode from this battle which made Black Star much stronger. However, he could only use it for a second before it drained him completely of his energy. So Stein gave him a special kind of training to take part in, where he would stick his hand in a jar filled with water that drained his energy. That way he would get used to the feeling and would eventually build up enough energy to last throughout it. He finally showed this power off for the first time against Free, however he could only use it for a brief time and fainted before even reaching the villain.
When the witch Medusa finally decided to put her plan into place to free Asura, the keshin sealed under Shibusen, she attacked during a Christmas Eve party and tried to seal everyone into the hall they were celebrating in. Luckily Sid used a transportation magic just before the attack to send Black Star along with Frank, Maka, and the rest of the protagonist of the series out of the room. They ran across Medusa and Black Star, Kid, and Maka were able to make it past her while Frank battled with her, leaving the three of them with the job of stopping Asura from being injected with black blood that would resurrect him.
Black Star tries to put Asura down as soon as he gets up
Black Star eventually caught up with Kid who had moved ahead and was in the middle of fighting Free. However they eventually discovered that Free was just an illusion and he was actually much further ahead of them. Finally they reached Free and Eruka Frog, who were already in the chamber holding Asura. Black Star tried to slice the needle holding the black blood in half, however it turned out that he was hallucinating due to the effects of the madness that simply being in Asura's presence can create. He actually sliced through a statue while Eruka successfully injected Asura with the blood, resurrecting him while Black Star was so exhausted from the power he put out, using Tsubaki's cursed sword mode, that he could barely stand. He attacked Asura with his Big Wave attack but he couldn't do much damage to him and was smacked down quickly afterwards.
Black Star suffers a great defeat at the hands of Mifune
When Shibusen learns that the witch Arachne is plotting on creating a device that can change someone's morality, inducing insanity into them, Sid and the rest of his Black Ops team heads out to destroy the facility creating it. However, Black Star overhears this and he decides he is going to set out as well and destroy the facility himself in order to make them pay for hurting Maka. When he arrives at the facility, he finds that Mifune is there, he is working for Arachne because he is sworn to protect Angela and she is currently part of the group of witches that she has gathered, even if Angela is too young to understand what is happening. Soul says this time he won't let Mifune walk away, however Mifune isn't kidding around this time and fights seriously. Black Star is quickly defeated, even with his new cursed sword mode.
Mifune lets Black Star live, and tells Tsubaki to take him away for treatment. She says he should come with them in order to become a teacher at Shibusen, but he finds the idea absurd. Before leaving he tosses Black Star a piece of candy, one of Angela's favorite kinds. When he returns to Shibusen Black Star hands the candy off to Maka and then sits under the stars, thinking about the defeat he just suffered.