later in volume 12 it is revealed that 4 of them are Mutation Pieces
King (Independent) of Hyoudou’s Peerage
King of ‘Red Dragon Emperor of the Blazing Truth’ team in Azazel Cup
until Volume 11, becomes Humanoid-Dragon in Volume 12
Dragonification of body parts
As the story progresses, its revealed the reason of he getting attracted by other girls is a side effect of his Boosted Gear and Dragon Power. Since more girl attracted by him, he often getting annoyed by girls which always fight over him. Starts from volume 18, after Michael crafted a special room which is located in a different dimension and give the door-handle-like key to Irina, she and other girls decided to take a turn for use it and force him to have a sex inside that room. Which makes him more aware at night.
In a middle of volume 22, Issei get promoted as High-class Devil according by his achievements in one year (Take down Loki, overcome Bandersnatch, Jabberwocky, and even Trihexa attack, confrontng Khaos Brigade and Qlippoth’s strike, Fatally-injure Rizevim, and Defeat Evil Dragons Grendel and Apophis). Since its a very rare case that a reincarnated devil became a high-class devil only in 1 year from his reincarnation and his achievement on ‘War of the Evil Dragons’, Issei became a public Highlight and became more popular in Underworld and other Myth’s world as ‘Red Dragon Emperor of the Blazing Truth’.