Exactly what is Replica
Yes, the universe has barely begun to open up, but I have to wonder.
Replica is introduced as a Trion Warrior that acts as Yuuma guardian. However, it/he never really goes so far as to do that. Replica often allows Yuuma to go and do dangerous things and often acts as a mere sounding board for Yuuma ideas. True, Replica offers warnings when necessary but never (as far as I remember) outright directly stops him from doing something.
Replica is basically autonomous. In the Invasion arc, Replica sacrifices it/himself for Osamu sake, as well as Border as a whole. While you could argue that Yuuma had given Replica the initial order to help Osamu, this self sacrifice seemed to extend past a simple man to machine command, at least to me.
Honestly, I wondering if Replica is really just a Trion Warrior. I beginning to think that Replica is actually some manifestation of Yuuma father, Kuga Yuugo. In other words, perhaps Replica is a second Black Trigger that was created when Yuugo sacrificed himself. The first, the ring Yuuma carries, contains Yuugo power, while Replica contains his will. modeled after Yuugo himself. He created Replica initially for data storage and for something else, which has yet to be touched upon. Replica was made with trion, but he was also programed by Yuugo himself, which could mean that he made Replica in such a way that it works like a warning system but not a prevention system. Yuugo himelf knew that Yuuma was a stubborn child, and would rather do what he wanted instead of following orders. Which is what got him killed in the first place. I think Yuuma knows that his father did something to replica to give him autonomy, and I think he feels strongly about his father death which is why he was so attached to Replica. In the end, I think Replica served as surrogate father to Yuuma, while holding a vast amount of information of not only Yuugo,
but of his travels, and of Yuuma travels as well. After all, how did he know that replacing Yuuma dying body with a Trion replacement would work unless he had concluded it previously? I wouldn go so far to think that Yuugo predicted that Yuuma would get himself maimed, but it was probably a big “maybe” on his mind that he wanted to make a contingency plan for. Thus, Yuugo designed, programmed, and tested Replica as well as whatever unknown Trion experiments, so that if the time ever came, he could sacrifice his body to save his son while siphoning and saving both his “soul” and his strength somewhere so it would not be wasted. Yuugo did seem like he knew a lot more than he let on, even to people he cared deeply for. He instead put that data into Replica, which was then transferred to Border when replica gave them that data during the “war” arc. In this way replica, I think, fulfilled his duty to Yuugo to transfer all the data to Border, which Yuugo was originally a part of. This also helps to explain how Replica helps to use Yuuma seals. If I remember correctly, Yuuma can use the seals, but not past the 1x as he is not very skilled with it. However, with Replica he can go beyond 1x, and because replica is a part of Yuugo, he can push the trigger beyond that of it normal limits. I pretty sure he had a fair share of knowledge and connections in order to do so. Seeing as how he had great ideas for ranking wars, replica, maps and data, he was definitely someone who does a ton of research as well as experimentation. I honestly think it will be hard to go into how Yuuma Black trigger works as most triggers require trion to be pumped into instead of the ring which makes a trion body for Yuuma himself (I didn remember where he was damaged in the flash back but it seems unless as long as you have a trion organ and brain you can kind of extend your life with that ring for a longer period)
Replica on the other had is basically just a computer warning system during the first fight against Miwa we saw that he can analyze the weight bullets and informs Yuuma of how the bullets work. Replica rarely ever makes deciding choices for any of the characters,
he typically end his sentences with “I can make that decision for you” the only few times he does decide on actions is when he is certain that someone will be in danger( Note: Creation of a rabbit) As for seals i sure Yuuma can use them with ease it was never mentioned he required replica in order to cast them. Early on in the story he ask replica to use seals but that might just be because that they weren in boarder and maybe the seals doesn require as much trion because i faintly remember there a limit to seals after the fight with the cane user. All in all replica is just a warning trion soldier which gives ideas analysis and warnings to his owners. Fyi we replica should have a limit of trion(and we don know how to pump more trion into it) so i honestly don think Yuuma requires replicate to make seals.