Every great commander has a great second-in-command watching their back. Whether it's for a righteous cause or villainous ambitions, they will stick with their leader through thick and thin. They may be so devoted because they sincerely believe in the boss' cause, like Riza Hawkeye in Fullmetal Alchemist, or because they deeply admire their master as a person, like Robert E.O. Speedwagon in JoJo's Bizarre Adventure.
Loyalty and determination are highly lauded traits in anime, and so the characters who exemplify them are often fan favorites. The motivations behind them can be as interesting as the characters themselves, and the truth is, their bosses would be nothing without them.
Hak (Yona of the Dawn)
Known throughout the kingdom of Kouka as the Thunder Beast for his legendary strength, Hak has been Princess Yona's friend and bodyguard since childhood. When Yona's cousin Su-won murders the king and steals the throne, Hak goes on the run with Yona to save her from the same fate.
Hak doesn't stay with Yona only out of duty: he is deeply in love with her, and will do anything to protect her. He is capable of defeating throngs of enemies and powering through serious injuries to keep her safe, and takes great pride in watching Yona become a stronger, more capable leader.
Legato Bluesummers (Trigun)
Knives Millions didn't intend to save a young Legato's life, but save it he did. Now Legato devotes his life entirely to Knives, and shares Knives' goal of eradicating all humanity, himself included.
One of the most underrated anime villains of all time, Legato commands Knives' other followers while the boss himself lurks in the shadows. He takes twisted pleasure in making Knives' brother Vash suffer eternally for abandoning Knives, to the point of pushing Vash to kill him and betray his pacifist ideals in the episode "Sin."
Riza Hawkeye (Fullmetal Alchemist)
Lieutenant Hawkeye serves in the military of Amestris, but her first loyalty is to her boss, Colonel Roy Mustang. After both taking part in the horrors of the Ishvalan War, Riza is Roy's staunchest ally in his plan to overthrow the corrupt government of Amestris and atone for what they did.
Riza is the one who gave Roy the array for the Flame Alchemy he is famous for, starting off one of the most touching friendships in Fullmetal Alchemist. She protects him both from harm and from himself: she promises him that if his ambitions corrupt him as well, she will kill him herself.
Nui Harime (Kill la Kill)
When Ragyo Kiryuin's attempts to fuse her daughters with Life Fibers after birth failed, she resorted to growing a human-Life Fiber hybrid in an artificial womb. The result was Nui, a terrifyingly powerful being that serves at Ragyo's whim. Nui adores her "mother," and takes sadistic glee in tormenting and killing their enemies.
Though being permanently damaged is normally enough to send her into a frothing rage, such as in the episode "Tell Me How You Feel" when Ryuko severs both her arms, this does not apply to Ragyo. When Ragyo's ultimate plan requires Nui to die and fuse with the Primordial Life Fiber, she cuts her own head off with a cheerful "Oui, Maman!" ("Yes, mom!")
Joey Wheeler (Yu-Gi-Oh!)
Yu-Gi-Oh! is a series that runs on the power of friendship, and there is no more loyal friend to Yugi than Joey. Though their relationship began with Joey bullying Yugi, Yugi standing up to a worse bully to defend Joey in spite of that sparked a complete change of heart. Now Joey always has complete faith in Yugi's abilities, and will hot-headedly jump into action to protect him if he is threatened.
This is most impressively shown in the episode "Friends 'Til The End, Part 4," when Joey first breaks free of Marik's brainwashing through the strength of his bond with Yugi, refusing to hurt his friend in one of Yu-Gi-Oh's most wholesome moments. On top of that, he then nearly drowns saving Yugi's life.
Konan (Naruto)
Konan is the strong and stoic right hand woman of Akatsuki's leader, Nagato. She trained and fought alongside Nagato from when they were children, and was one of the founding members of the organization. She protects his original body and carries out his orders, earning the title of "God's Angel" from their followers.
She believes wholeheartedly in Nagato's vision for world peace, and after Nagato's death, places her faith in Naruto to fulfill it in their place. In the episode "Hero of the Hidden Leaf," she tells Naruto that because Nagato believed in him, she will believe in him as well.
Robert E.O. Speedwagon (JoJo's Bizarre Adventure)
Speedwagon starts out as a criminal from the worst part of London, killing and mugging people to survive. Everything changes when he targets Jonathan Joestar, who beats him in a fight then spares his life. He is so impressed by Jonathan's nobility that he becomes Jonathan's most steadfast supporter forever after.
Speedwagon's loyalty lives on long after Jonathan's premature death. In Battle Tendency, he's shown to be a treasured friend to Jonathan's son George and grandson Joseph, and the Speedwagon Foundation he established provides invaluable resources to the Joestar family.
Giriko (Soul Eater)
Most weapons in Soul Eater are self-sacrificially loyal. Despite not needing a meister to wield him (and proud of it!), Giriko is no exception to the rule. He is, however, the only one who can boast that his devotion has lasted for eight whole centuries.
When his master Arachne lost her body, Giriko guarded her soul, surviving by transferring his memories and soul into his many children. Upon Arachne's revival, he is her strongest soldier. In the manga, when she is killed permanently by Maka and Soul, he finally dies trying to avenge her, seeing no purpose in life without her.
Ja'far (Magi: Labyrinth of Magic)
In the world of Magi, a King Vessel is bolstered by their many Household Vessels, warriors empowered by loyalty to their king, an aspect of the anime similar to Justice League. Ja'far is one of King Sinbad's Eight Generals, and the second-in-command of the entire kingdom of Sindria.
A former assassin reformed by Sinbad's friendship, Ja'far is willing to kill anyone who threatens Sinbad and stands by his king even as Sinbad's methods become more underhanded. In the manga's final chapters, when Sinbad merges with the Rukh and disappears from the world, Ja'far is left to lead in his place, loyally waiting for his return.
Nora (Noragami)
Nora is a stray Regalia who was raised alongside Yato by their "Father." She has had many different divine wielders, but always returns to Father, ready to give him inside information on the gods and happy to do his bidding once again. Though he views her as little more than a tool, she is constantly trying to earn his love.
She takes particular joy in helping Father corrupt and isolate Yato, who is attempting to break free of their control. She wants to recreate the "family" the three of them once were, with herself as Yato's only Regalia, and is willing to adopt Father's manipulative ways in order to bend him to their will.