User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-104.7.80.176-20150224020838/@comment-75.71.34.254-20150304182445

Irregular probably refers to the combination of exact opposites. I only say this because Issei is referred to as having irregular growth several times. Although I think Vali may have been referred to the same once. But I'm thinking about the announcement at the end of chapter 12 when Azazel announced he was stepping down as Govener: "the current Sekiryuutei who shows irregular growth and the current Hakuryuukou who is said to be the strongest in history"

Maybe I'm reading a parallel that doen't exist.