Erika Ueno (ygo 5Ds)

Erika is the biological daughter of the Ueno family which is specialized in producing D-Wheels for Security Sectors. 

She also has an older adopted sister named Suzume.

Suzume is a satellite that was adopted by Erika's parents when she was still a child; her adoption was authorized by Rex Goodwin who took an interest in Suzume as she holds with her a card named "Fragment of Death", which was believed to be the fragment of a forgotten miraculous source.

Erika was born 5 years after Suzume's adoption.

Since Suzume and Erika were going to inherit the family business in the future, Suzume was enrolled to Duel Academy to become a good duelist - and a Sector Security in the future - while Erika was taught mechanics so she became good at build up and repair D-Wheels. 

Suzume and Erika have two opposite personalities: while Suzume is more of a solar girl that easily gets scared, Erika shows a colder attitude and rarely smiles. Erika is also very stubborn and doesn't care about danger. She is also too young to actually be able to drive a D-Wheel, though she built one in secret. Once the D-Wheel was ready, Erika decided to try and drive it, however she lost control of it and was caught in an accident. Once at the hospital, Erika was given dead by the doctors, however Suzume's prayers to the miraculous fragment card managed to save Erika. As a lucky charm, Suzume decided to gave Erika her "Fragment of Death" card wich seemed to match with Erika's new ace, the Lord of the Underworld, the Snake Skeleton King.

Erika's recovering was seen with suspect by people like Goodwin who interrogated the girl, the latter not giving him any satisfing answer. Eventually Erika went missing after a bit of time as she was indeed kidnapped by the Dark Signers as they believed she was one of them and possibly the reincarnation of the "Fragment of Death" as well - but is she? 

Truth is that Erika is a kind of "Dark Signer", in fact she definitely was dead after the accident that accoured though she wasn't revived by a Earthbound, but by the Lord of the Underwold, the Snake Skeleton King which heard Suzume's prayers and revived Erika as the vessel of the "Fragment of Death", choosing her to give birth to one of the incarnation of Kiseki's Mirror of Miracles; Erika usually have deep conversation with her ace monster

As for Suzume, Erika's disappearance made her crazy as she left Duel Academy in search of her younger sister. She travels back to the satellite after hearing rumors of a young boy that was solving cases down there. She was determinated to find that guy ready to pay him any amount of money in exchange for his help in order to find her sister and make sure that she was save and sound.