Like all Ex-machina, Einithi reassmble an young and attractive Imanity male with red hair except for having machanical part spring out from his body. In the light novel, he had a scar that cut across his nose as courtesy of 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 wears a funeral dress-like black cloak when he first address himself to Riku Dola to bring an end to the ancient war. However, by the time of his re-appearance in Elchea Kingdom, he dressed himself in a same-coloured yet more casual butler outfits, only wear the cloak in traveling.
As the Ex-machina have a shared mental connection, he refers to himself or themselves as "worse than mindless machines".. "Einithi" is a callsign for him and his predecessor unit 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, Einithi acts like a cold, emotionless figure to accept the self-sacrifices of his brethren. He buried his mourn deep in his heart to stop the war, but actually cared about his clusters and could name individuals.
It was revealed to Sora that due to reflecting Shuvi's feeling toward Riku, Einithi had become homosexual and attracted 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.