Tower of God has regularly been compared to shonen classics, especially Hunter x Hunter -- and for good reason. Even though both series more or less follow the shonen formula, both play with genre conventions by using a battle system that relies heavily on tactics and intelligence rather than brute force.
This aspect is best reflected in both series' deuteragonists, the fan-favorite Killua Zoldyck from Hunter x Hunter and Khun Aguero Agnis from Tower of God. Despite being largely unrelated, these two characters have a shocking amount of similarities. Both have blue eyes and light hair, but more importantly, they have similar family backgrounds which lead to similar personalities, and even a similar relationship with their protagonist.
Killua and Aguero are even introduced in a similar fashion, meeting the hero through a grueling tournament to qualify for bigger adventures ahead of them. The Hunter Exam and the Testing Floor have similar purposes, they are setup for the world and a ground for the main characters to meet and form relationships. It is through these exams that Killua and Aguero first became intrigued by and then befriended the protagonists Gon and Bam, whose naïve and straightforward personalities are unlike anyone they’ve ever met.
Killua and Aguero’s upbringings explains why they are drawn to Gon and Bam. Killua grew up in a famous family of assassins, the Zoldyck Family, and is rigorously trained to be the heir. He is taught to be ruthless and cautious from a young age, and his brother Illumi even stuck a needle in his head just to make sure of it. Aguero grew up in an even more brutal world, as a member of the extremely competitive Khun Family. Despite his intelligence, Aguero is banished from the family after his own sister failed to become a Jahad Princess, so he is understandably cynical and deceitful. But their dark pasts made them crave a friend they could trust without reservation, namely Gon and Bam.
Killua and Aguero are important foils needed to help our inexperienced protagonists navigate the cutthroat worlds of the Hunters and the Tower respectively. Without their intellect and knowledge, Gon and Bam would not likely have passed their first few tests, let alone advance to where they are now. They are also some of the stronger characters at the earlier stages of the series, thus they are both quite arrogant in the beginning. Killua is a gifted assassin and had already killed dozens of people before the age of ten, while Aguero is from one of the Ten Great Families, which alone makes him superior to most Regulars, but his resourcefulness and strategic abilities are almost unrivaled by his peers. Their reassuring presence makes them incredibly dependable teammates, and they can be truly deadly when they need to be.
Compared to the protagonists, Killua and Aguero are actually the characters that receive more development. Since Gon and Bam both have unwavering convictions, as shonen protagonists often do, their basic personalities don’t change that much even after all they’ve been through. Whereas Killua and Aguero undergo some major growth thanks to meeting the protagonists, they become better people and more sophisticated characters. They need their best friends to keep them grounded and help them see the good in the world, while still able to retain their worldliness and cunning. Under different circumstances, they could have easily become villains of the story, but their occasional villainous qualities are also a part of their charm.
As a result, one of both characters’ defining qualities is their devotion to the protagonists. Killua’s loyalty to Gon culminates in the Chairman Election arc, where he basically had to go up against his entire family to save Gon’s life. And Aguero had to carry Rachel, someone he despises, for six years just because he made a promise to Bam, whom he thought had died. This level of sacrifice makes them the absolute best boys of their series.
All this is not to say that they are carbon copies of each other, actually far from it, they each face their own unique trials and tribulations. For one thing, Killua is a loving and caring big brother to his sister Alluka, while Aguero and his sister Khun Kiseia’s relationship is fraught to say the least. Not to mention all the other life or death situations they had to experience over the course of their respective series. Their similarities are what initially attract audiences to them, but their individual journeys are what make fans eventually fall in love with them over time.