Like all Ex-Machina, Einzieg resembles a young, attractive member of Imanity with some mechanical parts exposed. He himself looks like an Imanity male with striking red hair. In the light novel, he has a scar on his nose he got during the Great War. Despite being more than 6000 years old, comparable to non-aging Jibril, he looks only slighty older than Sora in the latter's view.
He initially wore a funeral dress-like black cloak when he first addressed Riku Dola to bring an end to the ancient war. However, by the time of his re-appearance in Elchea Kingdom, he changed his outfit by dressing in similarly coloured yet more casual butler outfits, only wearing the cloak when traveling.
As the Ex-Machina have a shared mental connection, he refers to himself or themselves as "worse than mindless machines". "Einzieg" is a callsign for him and his predecessor units rather than an individual's given name. Originally blunt and cold, he later feels warmer about himself and the Ex-machina after Shuvi programs feelings into him and the cluster, causing him to feel grateful.
Much like Riku, Einzieg acts like a cold, emotionless figure to accept the self-sacrifices of his brethren. He buried his grief deep in his heart in order to stop the war but actually cared about his clusters and could even name individual units.
It was revealed to Sora that due to reflecting Shuvi's feelings toward Riku, Einzieg developed a homosexual attraction to the Imanity king due to his striking resemblance to Riku. He frequently called Sora affectionate pet names and was even able to find his entire hentai collection.
"Einzige" isn't a name for the male ex-machina, but a title for the administrator unit, which mean Einzige inherited his title and role from his predecessor and can possibly pass it to next administrator unit.