Nanahoshi Shizuka's friend and one of the two people Rudeus was not able to push out of the way from the truck before he died in his previous life.
He did not actually appear in the volume, but was mentioned by Nanahoshi. Nanahoshi created a hypothesis that he was also summoned to this world, but was summoned into the future where he saved a weak fated Miko that was destined to die. That Miko had the power to rewind an object's time and Akihito possibly saved that Miko and met with Orsted as Nanahoshi did to create a device that would amplify the Miko's power to change the past. If the future is changing the past, the future would take precedence and be the reason why Nanahoshi can't go back to her world since she may have been summoned by Akihito during the Metastasis event so that she can create the Multiverse teleportation device and go back to her world with him in the future. Since her future self that went back with Akihito can't exist if she went back to her world in the present, Akihito's influence interfered with the device when she was attempting to go back home.
In Rudeus's inner monologue, he summarized the details of the proposed future. Akihito collaborated with Orsted and was able to determine the reason why he would be unable to win against Hitogami. Akihito altered the past with the saved Miko's amplified power which may have been the reason Hitogami saw a vision of Rudeus's descendants with Orsted and an unknown man killing Hitogami and that the unknown man might be Akihito. Then in order to go back home, he caused the Metastasis event that summoned Nanahoshi so that she would create the device that would send them back to their world.
Epilogue: Prologue Zero
In the most recent life of the Rebirth Miko, he was summoned by magicians using a magic formation that tapped into the Miko's powers as a Hero to save the Kingdom from war. After being summoned, he was the only person in all of the Rebirth Miko's lives to have ever asked for her name. Since a Miko are only called Miko as their name and lose their actual names (Unless they are Royalty), the Rebirth Miko is a special case that has remembered her name to be Riria. Since Riria the Miko was the only one capable of understanding Akihito, she was assigned to be his interpreter and freed from her usual constraints as a Miko. He without knowing has made Riria fall in love with him. During the battle with an enemy Kingdom, he was instantly beheaded and by the time the Riria found him, 24 hours have already passed and it was too late for her to revive him. In her despair and wishing with all of her power, Riria opened a rift in time and space in the past so that Akihito and her could not just live, but to live together. That rift created an opening for Rudeus's soul which happened to be near Akihito in the other world to be reincarnated. More of Riria's power was exhausted to culminate in a rift that caused the Mana Calamity that summoned Nanahoshi to the world. As a result of Akihito meeting Riria in a previous loop, he caused a change in the world that resulted in the events of what happened in Mushoku Tensei and the exhausting of Riria's Rebirth Miko powers and Riria's last life.