Like many of his fellow soldiers and teacher at DWMA, Sid is a capable fighter who is loyal to his teammates and students, although prone to some eccentricities. Strict with many of his students and, after his transformation into a zombie, frightening to some of them, Sid continues to mentor these students, in life and in death, especially his Central Intelligence agents Akane☆Hoshi and Clay Sizemore. As a meister, he is a suitable partner to Mira Naigus, as well as capable of wielding Death Scythes such as Azusa Yumi and Spirit Albarn. As a trusted member of Death's staff, he has frequent one-on-one meetings with the head of DWMA, making him a fierce opponent against Arachnophobia and any of Death's other enemies.
Before and after Sid's death, Mira worked well as Sid's weapon, both in combat and in intelligence gathering.
According to Sid, the two became partners upon meeting at an orientation. The two have taught separate classes for N.O.T. and E.A.T. students in the classroom and in physical education, while collaborating on intelligence gathering.
Following Sid's death, Mira expressed sadness at his loss. Upon his resurrection as a zombie, Mira seemed to still regard Sid not differently, although her battle attired changed to include more bandages.
Mira is rather blunt when contending with Sid's foolishness. When Sid expressed dismay at reading Medusa's journal, she chastised him that his chivalry is less important than proving she is a witch. Yet she had no obvious reaction when Sid referred to her new nurse uniform as "sexy."
Flashbacks in the manga show that Sid adopted Black☆Star when he himself was barely a teenager. Sid therefore has a close relationship with Black☆Star, although little connection is shown in the anime. After the DWMA assassinated members of the Star Clan, Sid was the person who adopted and raised the last survivor, the infant Black☆Star.
Sid is at times exasperated dealing with Black☆Star, preferring to give the student strict instructions to follow rather than negotiating work. Unfortunately, when dealing with someone as irresponsible as Black☆Star, it is more likely that the student will disregard Sid's instructions anyway. For example, when frustrated at Black☆Star's failure to capture one soul despite how hard Tsubaki Nakatsukasa was working, Sid gave Black☆Star a remedial lesson to clean the entire library at the DWMA. Despite Black☆Star's protests, Sid simply ignored the student and left the library. Therefore, without any supervision, Black☆Star didn't bother cleaning the library, instead reading comics for much of the day before leaving Tsubaki to do all the cleaning while he left for the Eternal Cave to acquire Excalibur.
Despite Black☆Star's rudeness to Sid, the two do share a strong relationship as student and teacher, maybe even son and father, as it was Sid who taught many of the skills that Black☆Star demonstrated early in the series.
Working for Central Intelligence, Sid supervises Akane and Clay, who have worked for the organization both as undercover bodyguards and as EAT fighters. Sid trusts Clay as an expert in intelligence gathering and Akane as both a skilled actor and capable meister. Even if Akane is prone to mock Clay, Sid speaks openly about how pleased he is with both students' work for Central Intelligence. Sid is so confident in the two students that, when preparing for the fight against Asura on the Moon, he approaches both students directly to bring them on the mission.
During the events of Soul Eater NOT!, Sid knows Akane and Clay are working with him undercover as NOT students to serve as bodyguards for Anya Hepburn and to investigate reports of Traitors. Therefore, as a NOT instructor, Sid covers for the two students when they are late or missing from NOT classes, playing up the charade when Akane claims he is late for class due to Clay wasting time at an arcade.
When Shaula Gorgon kills Sid, Akane and Clay are both deeply troubled by his demise. While Akane maintains a more steely resolve when attempting to convince Anya Hepburn to leave Death City to avoid any danger during Shaula's attack, Clay is more obviously emotional, yelling at Anya that if Sid was alive, he would not want to see her endangered.
Sid shares a deep respect for Death and greatly takes pride in himself knowing that he can always help him on dangerous missions that lower starred meisters cannot handle. Sid is one of Death's most trusted DWMA staff members, and Sid is mutually trusting of Death in every way.
As a whole, Sid's relationship to Death bears much similarity to Spirit's, Death's own Weapon partner: a bit of friendliness, but also surprise at how informal the shinigami can be. As Sid has a friendship with Death, the two will converse with each other just as often as Spirit does with Death, shown in the anime as sharing tea in the Death Room or through the mirrors in the men's bathroom. However, Sid can be surprised at Death's lackadaisical nature, as Death is easy to forgive potential traitors such as Crona and witches like Kim Diehl and Angela Leon, and Death is too candid about some secret information, such as revealing to almost the entire Academy that the Kishin is locked up in the basement.
Stein expressed some surprise when Sid recovered the infant survivor of the Star Clan, Black☆Star, but seems to have accepted Sid adopting the baby. During their youths, Stein and Sid had, likely on separated occasions, attempted to meet with the Holy Sword, Excalibur, and the two commiserate in front of Tsubaki Nakatsukasa about how miserable it was to deal with that annoying weapon.
After Stein left the DWMA, Sid frequently spoke with Stein, attempting to convince him to return to the Academy as an instructor. Such appeals seem not to convinced Stein, until after Sid's death when Death approached Sid himself to ask that the scientist investigate reports of a witch who was experimenting on a certain meister. How much Sid's death and his previous attempts to convince Stein to come to the Academy is unclear.
Stein is also one of Sid's trusted physician. When Sid died, his corpse was brought to Tsugihagi Research Laboratory before being ostensibly buried in Hook Cemetery. It was likely during this time that Stein turned Sid into a zombie, per request of Death in order to test Maka and Black Star in a remedial lesson. While Stein could not return Sid back to normal, Sid does not seem to mind: while a significant change for Sid's life, he seems to maintain a friendship with Stein, bragging to their students how great life as a zombie is, although this performance was likely to play the villain in the remedial lesson that Sid and Stein organized for Maka and Black☆Star. Stein may also have been the person who reattached or replaced his arm after it was cut off by Noah's Magic Worm.
Sid and Stein worked closely in numerous missions, including the investigation into Medusa's revival of the Kishin, the rescue of Death the Kid from Noah, and the fight against Asura on the Moon. While Stein shows concern about Sid and Mira facing a suspected witch like Medusa, Sid assures his colleague that he understands the risks. When Death issued a warrant for the arrest of Stein on suspicion of killing Joe Buttataki, Sid's experiences with Stein as a colleague and friend convinced him that the scientist did not kill Joe. Therefore, Sid allowed Stein to depart with Marie Mjolnir to locate the real killer.
When Sid died, Maka said that she always liked him as one of her teachers. Sid was Maka's teacher shortly before his death, and she would help him with his lessons when requested. For example, Sid called upon Maka and Soul Evans at least twice to teach his NOT students about practices in Soul Resonance, and Sid called upon Maka and Marie Mjolnir to meet with Crona to give the Academy's newest student a tour of the school.
However, Sid can be rather odd when dealing with Maka, and she in turn can be less formal, even violent, when dealing with her teacher. Sid performed his own Sid Chop upon her when he thought her lesson on weapon training was distracted by a conversation with Tsugumi Harudori. Also, when Soul Evans exposed Maka's underwear in a failed attempt to interrogate the zombie opponent, Sid was uninterested rather than as lustful as he was when Black☆Star lifted Tsubaki Nakatsukasa's skirt to show her underwear. Maka, in retaliation against Sid, as well as Soul and Black☆Star, hit her former teacher with a Maka Chop. Maka also insulted Sid's new zombie appearance by describing him to Crona as a "big gorilla."
As with all of his students, Sid is a committed educator, striving to teach his students to face the dangers of life as a meister or a weapon seriously. Therefore, Sid holds high standards for the trio and is blunt. He admonishes Tsugumi for being exhausted by the Academy's stairs, he directs Tsugumi to face her fears in her and Anya's duel against Hao and Raid,, and he gives her a severe grade for her failure to create a blade on her halberd weapon form. He also can demonstrate impatience with the students' indecisiveness, as when he interrupts Anya and Meme's negotiation over who will practice weapon training with Tsugumi to instead pair Anya with Aaron. Nevertheless, Tsugumi, Anya, and Meme seem to recognize him as a good teacher, largely following his instructions. Even when strict, Sid still will have a smile on his face and even pat a student's head for encouragement. The trio's familiarity with Sid leads Tsugumi and Anya to feel grief upon Sid's death, and causes Meme to feel guilt for her part in murdering Sid while she was under Shaula Gorgon's possession.
Sid also recognizes the importance of protecting Anya, as part of an agreement with the Yngling family to give her bodyguards while training at the DWMA, and to remove her from Death City whenever the city was under attack.
When Meme, possessed by Shaula, killed Sid, Tsugumi and Anya mourned deeply for their deceased teacher. How Sid and Meme related to each other after Franken Stein resurrected him as a zombie has not yet been shown.
As he relates to Tsugumi Harudori, Sid seems to be a stern teacher when addressing Kim. When she approached him about Hero's abusive and perverted attacks thanks to his new weapon, Excalibur, Sid simply replied that Kim had best grow stronger if she wanted to win against Hero.
Much like his relationship with Stein, Sid regards Marie as a colleague and fellow fighter. He is bemused that someone like Marie, who is a Death Scythe and student of the DWMA, still is lost in its hallways, but he is patient with her indirection. Like any of his colleagues, Sid is willing to defend her in combat: during the fight against Noah, Sid pushes Marie out of the path of a Magic Worm, saving her from potential injury at the cost of his arm.
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