Okay, so I am one of those who are disappointed in Pain's character. When I complained, people yelled at me "Write it better!" and "You do the manga then!".
So I did. I wrote my own version of Akatsuki Leader. I wrote his goals, his character, his power and his relationship with other Akatsuki members. I couldn't change everything, naturally. His appearance is the same as his first body in the original manga is.
His name is Asteren es Tereleth (a made up name). Only a few people knew this real name though, and just called him A'er'es. He lived 543 years back from todays Naruto timeline.
A'er'es was born in the very first shinobi village - a village who had the Sage of the Six Paths as their teacher. The world was in war, and the Sage was more than a human. He wanted to test his "godly" powers and teach the ninjutsu to humans. He started off from one village and ended up creating 3 different villages. After some time, he disappeared without a trace.
Those villages liked the new teachings and saw themselves superior to other human beings, and this led to a war. That war was a war of pure conquer and pillage.
As they found themselves inferior to the new power, other countries started researching ninjutsu themselves. Firstly, only the nobles had the chance to learn skills in the Academy one of the 3 new villages started.
Years passed, and every country sealed itself from the inside, researching the ninjutsu and expanding their techniques. Soon, they all forgot that there is a world other than the countries they know.
The area became isolated, and became known as The Shinobi World. That world was separated into countries by their nobles - The Land of Fire with their Hidden Leaf Village; The Land of Wind with their Hidden Sand Village; The Land of Earth with their Hidden Rock Village; The Land of Lightning with their Hidden Cloud Village; The Land of Water with their Hidden Mist Village; The Land of Grass with their Hidden Grass Village; The Land of Rain with their Hidden Rain Village; The Land of the Waterfall with their Hidden Waterfall Village; The Land of the Whirlpool with their Hidden Whirlpool Village and a few more Lands with their hidden villages.
Newely developed countries - click the spoiler.
These people bred wars through their history. After so much time has passed, some people considered being in peace, and actually living normally. They have even forgotten their purpose and the reason they were fighting.
That is the situation of todays ninja world - many shinobi don't like war and killing, but they are still doing it because they grew up in such an environment. See Kakashi, Obito, Naruto, Sakura, and many other characters in this manga - they are not all ready to take someone's life easily.
Don't tell me my plot has holes because Shodai created Konoha - let's just say that Konoha was destroyed and rebuilt many times, and that Shodai managed to bring Konoha the glory it once had and brought peace for a while.
Okay, so I explained the world stuff, time to get back to Akatsuki Leader, A'er'es.
Asteren was one of the very first shinobi ever. He was the genius of that time - he learned much faster than any other individual. For that, they despised him and praised him, from person to person. After a few years, 19 year old Asteren was way more skilled than anyone in his village, and he decided to start adventuring and test his mettle with other shinobi, whom he hoped will be more in the outworld of his village.
The lust for power and his arrogance led him to become an assassin, a merciless killer who was getting payed to kill. He took missions he wanted, and acted as he wanted. Many hated him for this and plotted against him.
One day, A'er'es came across the mystic mountain in his travels. He found the creature who was the link to all the summons there - a creature called The Shrike. It looked like this (click the spoiler)
As you probably noticed, the creature is very similar to the sealing statue.
This creature was brought to this world by the Sage of the Six Paths to serve as the medium for all the otherworld summons. The Shrike was imprisoned there, and it told A'er'es that he will gain more power than the Sage if he sealed The Shrike into himself, but to unlock his true power, the most powerful summons from the otherworld are needed - the creatures known as the Bijuu. If the Bijuu are all sealed in a special ritual, The Shrike will unlock its true power and become one with A'er'es.