My inner Sheogorath has decided! 

The choice for the way Rex will go is the Broken Earth! yay!! We have the demented version of a Rex that is both sane and insane and Lumine's path will be the blessed by dust path, meaning she will have Guizhong's blessing 

Preview: 

"Please, spin a thread of memory for me." A woman dressed in a tattered dress begged Lumine and the others as she carried something. A rook's base.

"Wow, Rein! I want to be like you one day!" Rex's voice echoed from the piece as Lumine gasped. They were inside a bubble universe where Rex sealed himself away to prevent himself from harming others.

"Kiana, here we are." Sirin Kaslana, the herrscher of the void said as she arrived via a portal with squad v and the Reve.

Reinhardt panted as he arrived. His combat uniform was tattered and he winced with each move. Numerous wounds slowly healed from his body as he dropped to a knee, only being supported by his weapon.

"Rein!" Lumine cried out as she helped her mentor stand up.

"Rex... Rex beat me. He almost killed me but he kept enough sanity to just... gah!" Reinhardt vomited blood before his eyes widened at the woman. the woman dressed in rags looked identical to their mother.

"I promised that I would be strong but I'm nothing." Reinhardt knew that voice! It was Rex's! It came from a broken chess piece.

"What is broken can never be fixed perfectly but it can be made better." The look-alike of the Crozzos' mother whispered as she presented the piece to them.

My inner madman has made me do something. 

The broken earth: The corroded lord of Geo

What happens when Rex becomes hated by the world save for his brothers? What happens when he is sent to study in St. Freya alone with the reputation of his ancestor, the oathbreaker weighing him deeply? What happens when each mission and almost every day he is given supplies that are either non-existent or barely even enough? 

What if Lumine and Rex never became friends and are just strangers and she even bullies him? 

The answer is a gun, a rook, and the inheritance of the title of the great dragon from his family's founder. Also, supportive Adoptive Gramma Fu Hua.

Basically, Ren takes Rex in and Fu Hua finds out. The three now live together and because Ren is a moron, he has to either share a bed with Fu Hua (risks of being castrated [temporarily] imminent) or sleep on the couch (a preferred choice)

Has lots of angst and drama

and has female genshin characters that have the medium female body type. 

Rex, before training with Ren:

Rex 'the punching bag/heart of Crozzo family,' Crozzo

Rex after receiving the geo Gnosis and training from Ren (and Fu Hua): 

(the horn is the corroded geo amber erupting from his body. He also has a prosthetic left hand. Ren is a brutal trainer)

Rex 'the corroded heart' Crozzo

Rex without his gear and using 'mirages' to hide his scars and prosthetics. 

Rex 'the demonic heart of the corroded desert,' Cruzzo

guns

blank-tinded Summit Shaper and Vortex Vanquisher