Jack was a child that wasn’t born out of love, his mother had him because she was impregnated by a man who she loved but didn’t love her back. The two lived in the slums together and Jack was born with a “gift” that would warp his mental state.
Kojirō lived his life how he wanted to do; everything he achieved was by his own hand. From the ability he used to slay a god down to the moment he died, Kojirō was a free spirit who walked to Earth by his own desire.
Okita was abandoned by his family and taken in by Kondō. He was the closest thing to family for him. Before their deaths, Kondō and Okita made a promise to each other that they’d always be by each other's sides no matter what, but unfortunately, Kondō had to sacrifice him for the greater good of the Shinsengumi, and because of that, Okita died alone without being able to fulfill his promise to Kondō. But to make matters worse, Okita did not die on the battlefield like he wanted, but instead died in bed due to tuberculosis.
Based off of what one of the leakers had said "The next human fighter is someone who has already been watching and following the previous fights" I had thought that Nostradamus might've been stepping up to the plate since humanity just suffered two loses in a row, Brünnhilde might've wanted to prevent the gods from gaining a lead by throwing out her joker. But based off of Okita's reaction from Leonidas's speech, I'm thinking it might be him next. Anubis vs Okita would make he most sense, in that case.