Siglai

"I like you, Sigma! Out of all of us, you're the one who acts the most ordinary when you're surprised!"

- Nikolai to Sigma in Chapter 78.

Siglai is the slash ship between Sigma and Nikolai Gogol from the Bungou Stray Dogs fandom.

Canon
Sigma and Nikolai are both members of the Decay of Angels. Not much is known about their first meeting or their interactions prior to on-screen events, however, Nikolai trusts Sigma enough to unveil his left eye and explain his plan to kill Fyodor. Nikolai is childish and dramatic around Sigma, teasing him and enjoying his reactions greatly, stating he likes how normal Sigma is. On the other hand, Sigma is wary around Nikolai, finding him odd and irritating; he states he hates dealing with Nikolai the most and appears unamused and annoyed by Nikolai's theatrics. Sigma has a level of respect for Nikolai despite harboring some dislike towards him; Sigma claims he has no doubt regarding Nikolai's capabilities, while Nikolai views Sigma as a valuable asset to his plans.

Kamui Revelation Arc
It’s not stated when Sigma and Nikolai first met, however, their first interaction shown in the series was when Nikolai saved Sigma as he was falling to his death from the casino. As Sigma fell from the sky awaiting death, Nikolai stands on the cliff and looks up laughing. Sigma wakes up with Nikolai standing by him, spluttering in shock and exclaiming, "Gogol?! Why are you here?!" before Nikolai grins and shouts, "Ahhh! This is great! That look of surprise! You're such a normal guy!", leaving Sigma sweating upon his rambling.

Sigma then notices how he doesn't have a scratch after his fall from the casino, and Nikolai states the trick is easy with a grin on his face as he explains how he used his ability to gradually decrease Sigma's fall and heal the wound on his chest. Nikolai shouts, "It's a bloody miracle!" leaving Sigma in silence, unamused and in thought. Sigma remains seated on the ground, examining his wounds as Nikolai swings his baton around boredly. Sigma mumbles to himself wondering why he was rescued by Nikolai why the former rescued him then retracts his statement, instead going on to ponder how Nikolai is still alive, given the events of Nikolai's "death" in the hostage situation.

Nikolai offers his hand to Sigma, telling him, "Why don't you ask me? I'm right here.", leaving Sigma with a grim expression and asking how Nikolai survived. Nikolai then puts on a "cute face" and says, "Oh, gee, how did I? Asking a magician to reveal his tricks is a bit rude, no?" teasing Sigma who glares and thinks to himself, "I hate dealing with this Decay member the most..." Nikolai then responds saying he's kidding, going on to say, "I like you, Sigma! Out of all of us, you're the one who acts the most ordinary when you're surprised! So, I'll tell you!"

Nikolai reveals his death was a mere illusion via his ability, leading Sigma incredulous, confusing Nikolai. Sigma then states that he knows Nikolai is capable of faking death since he's a "crazy, anything-goes" skill-user. Sigma asks why Nikolai faked his death contrast to the original plan that required his genuine death. Nikolai then takes off the card masking his left eye, smiling grimly while going on to explain how he intended to die but then remembered how he was understood by Fyodor, leading to his realization of wanting to kill Fyodor to be freed from human emotions. Nikolai laughs and announces he's going to kill Fyodor, and Sigma watches in horror, realizing how insane Nikolai is as Nikolai leans in and puts his hat over Sigma's head, stating he wants Sigma to help him find out Fyodor's skill.

Vampire Infection Outbreak Arc
Sigma and Nikolai devise a plan to break Fyodor and Dazai out of Meursault, though this occurs off-screen. The security goes into shambles as Sigma and Nikolai invade Meursault for the escape plan, and Nikolai uses his ability to free Fyodor and Dazai out of their cells. Nikolai gives a dramatic flair, announcing his arrival with Sigma as the latter stays back, watching with fatigue and annoyance. Throughout the scene, Nikolai happily explains how the plan worked, leading Sigma to suggest simply explaining the situation at hand, unamused by Nikolai's theatrics. Sigma watches Nikolai playfully reunite with Fyodor as the prison break game is explained. Nikolai claps his hands and giggles at Sigma's horrified reaction to the situation at hand.

Once Fyodor and Dazai begin injecting themselves with the poison, Sigma watches how the two along with Nikolai gleefully laugh, leading Sigma to ponder on how he's the only sane person in the room.

In Chapter 111, after Fyodor successfully wins the prison break game, Nikolai asks Fyodor what had happened to Sigma, growing grim after Fyodor informs Nikolai that Sigma would likely never open his eyes again, showing that Nikolai cares about Sigma to an extent

Fanon
Siglai is a relatively popular ship, rivaling Fyolai and Sigzai. Fans enjoy Siglai’s dynamic as Sigma is often irritated and tired by Nikolai’s antics while the latter enjoys teasing him. Shippers guess that since the two worked alongside another, they share many more interactions off-screen. The ship has around 2,000 works on AO3.

Fandom

 * FAN FICTION