Reference image: Here

Backstory on Vasto Lorde Akira:

When Akira died, he did not expect to see the one who killed him, Ryo Asu.... No.... Satan.... to break down and cry over his corpse. Satan has felt what he has felt over the death of his family, his aunt and uncle, Taro and of course, the one he loved and cared Miki And it was all his fault because he couldn't understand human emotions. Akira watched as the soul reapers tried to save the souls of both humans and devilmen who had fallen but it was to late, a horn was heard forcing the soul reapers to abandon all souls and return to the soul society. Akira was ready to accept the soul erasure that was going to happen. Soul Erasure happens when the angels, the higher authority of the soul reapers, come to burn everything and start anew. No reincarnation, no second chances, erase everything, save nothing. Akira was expecting this but what he did not expect was to be dragged into the world of the hollows before the angels arrived and burned everything.

Hollowfication was quick for Akira due to his short chain of fate and the fact that Amon, the strongest and most powerful demon was within him. But over time, Amon began to understand and feel what it was like to be human so when hollowfication happened, Amon gived all of his powers to Akira as his soul vanished making Akira the new Amon and the only Amon hollow in Hueco Mundo. In his short life of being a newborn, Akira made short work of any hollows that dare try to eat him. In one one vs many fight, the hollows and Akira himself merged into a Menos grande but Akira's heart took over the gillian form causing it to have a devil like mask. Akira started to devour numerous gillians and hollows until he became a Adjucha class hollow. Due to Amon's power flowing through him, he kept getting stronger and stronger until finally, he became vasto lorde and a powerful one at that. Pretty soon, Akira became the king of the hollows and they follow him and listen to him or else he would devour them. He also learns that Miki's soul was sent to the soul society before the angels came. The hollow king was relieved meaning that she can have a second chance once the Earth was restored and reborn. He also learned that Satan's soul survived the erasure and was trapped in his corpse so has punishment, the soul reapers sent the fallen angel to hell where is to stay forever more. As for Akira himself, he stays in Hueco mundo as the king, a Hollowman one would say. But he vowed that he will protect Miki once she is reincarnated and anyone standing in his way will pay with their life.

Akira Fudo belongs to Go Nagai
Hollows belongs to Tite Kubo