Sig Curtis is the burly, intimidating yet caring and doting husband of Izumi Curtis. Together with his wife, he runs a meat shop in Dublith and housed Edward and Alphonse Elric during their alchemy apprenticeship, though he himself is not an alchemist.
He met Izumi when she was 18-years-old somewhere in the northern regions of the country, and both immediately fell in love at first sight. Despite his perpetually angry-looking face, Sig is quite a nice person and laments the repellent nature of his visage. He is very devoted to and protective of Izumi, constantly worrying about her bad health and often needlessly rushing to her rescue when he senses danger (as he did when she was approached by Ulchi at Devil's Nest and when she was "stabbed" by Van Hohenheim) and becoming jealous when Izumi speaks fondly of other men. Sig has mentioned that he blames himself for not realizing sooner that Izumi had been experimenting with Human Transmutation.
Sig takes great pride in his significant musculature, pitting it against that of Alex Louis Armstrong. Seeing each other's "perfectly sculpted" bodies earned the others' respect. As a result, the two bonded in what Mason referred to as "a friendship forged from muscle". It is also shown that he may have had some amount of combat training, as he's easily able to catch a knife his wife casually threw at him between two fingers. Such interactions, Izumi's prominent combat ability and her declaration that her husband is a "finer man" hints that Sig's own level combat skill is at least a level his wife can respect and goes beyond just muscle.
After he and his wife marry, Izumi becomes pregnant with their first child. The two are joyed by this, but the baby dies during birth, causing the two to seep into depression. Izumi attempts to use alchemy to bring the baby back, but this results in The Truth taking Izumi's reproductive organs, leaving Izumi to suffer from Hematemesis and the two incapable of having another baby.
Before the story takes place, Sig, along with Izumi, run into a town threatened of being flooded. Izumi is able to easily save the town with alchemy, but vomits blood immediately afterward. They meet the Elric Brothers and Izumi teaches them alchemy. Sig plays as her voice of reason to keep the two from dying.
Meeting the Master
Sig and Izumi once again meet the Elric Brothers several years later when the two visit. Izumi welcomes them brashly, but Sig comes to her rescue when she vomits blood. The two catch up, but when the question about why they have not had children comes up, the two shrug it off and talk around the subject. Eventually, it is revealed to the Elric brothers that they cannot have children due to Izumi's encounter with the Truth, and that Ed's metal arm and leg and Al being in a suit of armor are a result of their confrontations with the Truth when trying to bring back their dead mother.
Izumi expels the Elrics and demands they leave her house. The two originally comply with this and leave, but Sig walks them to the train station and explains to them that Izumi now views them as equals. Realizing this, the two demand to stay so Izumi can help restore their bodies.
When Alphonse mysteriously vanishes, Sig, Izumi, and Mason investigate to find Al's whereabouts. They eventually find out that Al has been kidnapped by the Devil's Nest. Izumi goes to retrieve them, but is stopped by various (and some perverted) chimeras, led by Ulchi. Sig has apparently come along --to Izumi's surprise-- and gets annoyed by the chimeras frequent flirting. He punches one out and continues to beat them all, referring to Izumi as his woman (causing Izumi to blush with infatuation), and striking a manly victorious pose once he has defeated them all. Unfortunately, he is stopped by various smitten women and a few homosexual men, who all have a taste for men with muscles, as a result, he finds himself incapable of going any further.
King Bradley and Major Armstrong (the latter of which used tracking skills to follow the Elrics to Dublith) enter their shop and demand to know where Izumi Curtis is. Sig refuses to admit to anything, hiding it by telling them the prices of their meat. Armstrong attempts to persuade Sig by removing his shirt and showing him his massive physique. After seeing his impressive muscles, Sig decides to show off himself. By flexing all his muscles, he is able to tear his outfit to shreds. He then shows off his beefy body. Through continuous flexing and analyzing the others manly burl, the two form a friendship.
After the next confrontation, (which Sig and Izumi take no part in) Ed and Al leave for Rush Valley for Automail repairs.
The Human Philosopher's Stone
The two aren't again seen until midway through the story, when Sig and Izumi are on a train and once again encounter her students' father, Van Hohenheim. After leaving the train, they talk about Hohenheim's sons and what they have been doing when Izumi's internal injuries cause her to spit up blood. Concerned, Hohenheim then proceeds to reach inside Izumi's body, rearranging her organs. Thinking that he stabbed Izumi, Sig furiously punches Hohenheim, knocking him a few feet. Asking his wife if she is alright, Izumi maintains that she is fine, and that she can actually breathe better. Finding out that the wound where Hohenheim stabbed her is gone, he states that he cannot bring back her organs, but simply rearranged her damaged internal cavity in the most favorable arrangement possible. Izumi then fearfully asks Hohenheim what he is, to which he replies "I'm just a man named Van Hohenheim, the human Philosopher's Stone". After this, Hohenheim informs them both of everything that has been happening in the country, including the Promised Day as well as the Nationwide Transmutation Circle, and the Curtises head north. Once there, Izumi intentionally lets herself be captured by northern troops to inform a "trusted Ishvalan man" and a "large man with a Mohawk" of the Promised Day. This message runs through a trusted network of informants all across the country to reach Roy Mustang himself.
The Promised Day
In the final chapters of the manga, Sig decides to take an active role in helping the protagonists overthrow the dangerous Amestrian government. When Mustang begins his attack against Central, Sig and his wife both aid the Briggs forces against Central troops in the capital. Once inside Central Command, he and Izumi surprise Alex and Olivier Armstrong, as well as the various soldiers. He manages to surprise Alex by blowing the Homunculus, Sloth, several meters into the wall. Sig's example of strength, manliness, and muscle is enough to get Armstrong up to his full potential. Sig fights alongside Alex (who he still considers a good friend) against Sloth -- and with the two combined efforts, the Homunculus is eventually killed.
Sig and Izumi make their way down toward the lair of the Homunculi with the Armstrong siblings, hoping to do as much as they can to subvert the enemy's plan before Izumi must escape. However, when the five Human Sacrifices are summoned to Father's side, Sig watched helplessly as his beloved wife disappears into the Gate. After a good bit of searching and after the Nationwide Transmutation Circle is activated and reversed, Sig is happily reunited with Izumi just before the final battle begins. Unable to fight, he stays below while the alchemists and soldiers attack the godlike Homunculus leader on the surface, but at battle's end, he rejoins his friends, happy to see Izumi safe and moved to tears at the sight of Alphonse in his human body.
In the first anime series, Sig takes a considerably smaller role in the plot. He warily supports Izumi's attempt to reconnect with Wrath, though afraid of her being hurt in the process and implies that he had made the decision to abort Izumi's natural childbirth when it appeared that she would not have survived the labor. He was also shown to be able to comically sleep with both his eyes wide open.
- When Sig was younger, he greatly resembled King Bradley.
- He and Izumi can be seen in the background in Chapter 14, before their actual appearance in Chapter 19.
- In the 2003 anime, it is shown that he snores with his eyes open.