SPOILER my cousin read most of the light novel and from what I can gather, Elijah is also reincarnated from the same world as Arthur. I think Elijah is starting to remember his past life. In their past life when art was king Grey, i’m not sure what exactly happened but Arthur did something that resulted in killing the woman Elijah loved, Cecilia. Elijah hated king grey and what he felt right now was his hatred from when they were each others past selves. I’m pretty sure. :)
cecilia was captured by the government and elijah(nico) blames arthur then gets captured also, elijah escapes and warns arthur that they're using cecilia to rule the world cuz of her abnormal ki pool but Arthur didnt believe him and he went up against cecilia in the next round of the tournament and turns out the government was using elijah to blackmail her so she could never commit suicide so she let arthur kill her but elijah saw the whole thing and blames it on arthur even after he reincarnates...
i havent readt the whole volume 8 yet but i can arthur and elijah working together after finding out that he was used and that he only wanted cecilia to rule the world...

“Were you deprived of love in your past life or something?”
I’m cryinggggggg. If only our mans knew
It’ll make sense if u read the light novel