[n0m@n]

@larenthian: I will try to do the synopsis on BT then.

@Soul Assasin: Yes he's dense at first.

Spoiler for But...:

He realizes Asia fealing towards him first on volume 6. He realizes that Rias like him in volume 10 confessed to her in the end where she said okay. So from volume 11 it will be good. Also he's getting promoted in Top level Devil in volume 11 along with Akeno and Kiba. The devil peerage and level goes like this: Ultimate Level Devil>High Level Devil>Medium Level Devil>Low Level Devil. So basically he will become on par with Rias next volume. Also Rias's mother wants Isei to get married to Rias and she is okay with him making a harem. Even the other girls are okay with sharing Isei with each other. So he is becoming a Harem King.

@yuicchi:

Spoiler for Why Isei is considered a Dragon:

Well you know Isei has Sacred Gear "Boosted Gear" right. There was two Dragons which was always fighting against each other long before the story begin. They were called Two Heavenly Dragon. One was a red dragon and other was white. They were both stronger than God and they were being a nuisance to all three of the devil, fallen-angel, and the God (angels) faction. So all three factions teamed up to form a alliance to take down the two dragons where they were then sealed into two Sacred Gears, the "Boosted Gear" and "Divine Dividing". The red dragon, "Welsh Dragon Draig" was sealed into the Boosted Gear. The white dragon, "Vanishing Dragon Albion" was sealed into the "Divine Dividing".
People who posse the "Boosted Gear" is called "Seki-Ryu-Tei" which translates to "Red dragon Emperor". Every time the possessors dies the Sacred Gear gets passed onto a different person. But because Isei is always talking about breasts his nickname was changed to "Chichi-Ryu-Tei" which translates to "Tits dragon Emperor". He then became popular with all of the kids in the devils world and is now called "Oppai Dragon" which translates to "Breast Dragon".
He is considered to be a Dragon is well because of it. His balance breaker changes into a form of an Red scale-mail Armour which looks like a Dragon.

Spoiler for About Switch Princess:

Isei first uses the balance breaker in volume 2 which was still incomplete. He masters by volume 5 by pushing Rias's nipples. He gets "berserk" and unstoppable in volume 6 but reverts back to normal by pushing her nipples . In volume 9 his balance breaker gains new ability and he again pushed her nipples to get stronger. Yes the colour picture in Volume 9 has go to do with Isei getting a new form. And her breast were apparently glowing..... Rias was called "Crimson hair Rune Princess", but because of Isei her name was changed to "Switch Princess". Now both of them are called "Oppai Dragon" and "Switch Princess" respectivley. There is a tv show based on them at the devil's world. Though Rias was embarrassed and was crying sometimes because of it.....