Urouge the Fallen Monk

Urouge is a pirate who hails from a sky island and is the captain of the Fallen Monk Pirates. He is one of twelve pirates who are referred to as “The Worst Generation”

And his epithet “mad monk” is derived from 
Ji Gong 

the guy even has a movie after him

Birka 
Birka was a sky island far to the southeast from Skypiea. It is the birthplace of Enel and his followers.

Theory 
Enough of info, now let’s jump into theory. 6 years pior to current storyline Enel and his followers destroyed the homeland called Birka

Birka name is derived from ancient city of moon and there were 3 races of winged people
1.Shandia
2.Skypeians
3.Birka

After their arrival the 1st island they inhabited was Birka sky island. At some point in history, the three races became separated from each other and fell into the individual races. The Birkans settled on a Sky Island apparently named after their former home, Birka. They also appeared to “forget” their origins, although the moon, known to them as the “Fairy Vearth” remained a folk tale amongst their people, believing it to be a place of dreams.

I believe that Urogue was the God of Birka, now hold your horses, God doesn’t mean that he was almighty and powerful as we have seen Gan Fall God of Skypeia, an old man with no powers

Why Urouge was from Birka?
1.Look at his wings its similar ones Enel’s priests had

He dresses in similar robes to Divine Soldiers underneath the khaki robes he wears (which is also worn by the rest of his crew), which includes the white full-bodied garment with severed sleeves

2. Their name, Fallen Monk Pirates
Enel tried to destroy the Skypeia as people living in the sky is unnatural, so as God it is his job to keep the natural order. So we can assume he destroyed his homeland for same reasons ,and monks who survived the fall or escaped the island are Urouge and his followers hence named fallen monks pirates
Notice as divine soldiers and maidens with him and they follow him as their leader same as Gan Fall

3.His fear for lighting -After the war at Marineford, he and his crew go to the New World. They followed their Log Pose to a dark and stormy island that was spewing lightning, with no way to have a ship docking there
the raijin island

4. As captain of White Berets he said some fought Enel and and his followers but were unsuccessful they maybe fallen monk pirates

Scenario 2 :
This is short one, but one of possibilities is inspired by destruction real Birka

After having consistently described Birka as an existing city, Scholia 138 of IV 29 describes Birka’s sudden demise. Talking about Adalvard the Younger, the bishop of Sigtuna and later that of Skara, Adam or a later copyist has written:

“During his journey he seized the opportunity to make a detour to Birka, which is now reduced to loneliness so that one can hardly find vestiges of the city; therefore impossible to come upon the tomb of the holy Archbishop Unni.” (Scholia 138)
The remark does not make it clear if Adalvard found the city destroyed or if that had happened after his visit and the later remark was just to warn the future pilgrims not to go there anymore in vain. As Adalvard was back in Bremen already by 1069 and is mentioned as one of Adam’s sources of information, it would have been expected that word about Birka’s destruction had reached also Adam before he published his work half a decade later.

Urouge and his followers returned home only to find their home wiped out without a trace and hence left for blue sea, i find this unlikely but worthy enough to mention as inspired by real events is one of Oda’s thing

Urouge’s future role 
I believe that winged races (shandia,skypeia and birka) were once allied with Ancient Kingdom
For now Luffy saved both shandia and skypeians from Enel and his followers and took payback for birka ,which Urouge doesn’t know, it’s very similar to Laboon and Brook, Luffy stopped Laboon from damaging himself later, Brook was thankful for that and at some point of time Urouge will ally himself with Luffy similar to shandia and skypeia people
Side note: Urouge might meet Strawhats at Elbaf as the land ruled by God theme is similar to Skypeia

*Theory by Dave