User blog comment:B214/Question & Answers 5/@comment-67.211.8.19-20150423133545/@comment-16281343-20150423134404

His real name is unknown. He was given the name Isaiah when he was a test subject. As you've read in v15 (short story "May The Shine Be On You"), he discarded that one and refused to use it. That's why he was given another one by Rias; Kiba Yuuto.