Since I read it in the manga, Railgun's Dream Ranker's Arc was one of my favorite arcs in the whole #Raildex franchise. The arc is compromised of different sub-stories that are only connected by special cards letting people share dreams: India Poker. The arc is named after the people who produce such cards: Dream Rankers.
Spoilers Up to Railgun T Episode 22
Wrote this as a Manga reader, but no spoilers for episodes not shown in anime yet.
Dream Ranker's arc first introduces Indian Poker cards as a fad for Academy City's students. They soon learn that the cards allow the user to learn the skills they dream about. (For example: Someone learning an entire new language in one night.) People with exceptional dreams started selling theirs for profit. Scientists started using these cards to backup their "intelligence" in a physical form.
The first story of the arc is how Misaka got introduced to the cards. The second story stars Kuroko as she tries to prevent a disaster predicted by a treasure hunter app that somehow shows future events. The third and fourth stories each star a member of ITEM meeting Misaka and Saten respectively. Indian Poker play a huge role in all of these stories, even if they were only there in the background.
The final story starts halfway through the arc, we get introduced to the creator of these cards: Kuriba Ryouko, a young scientist, (still in the middle school,) who is researching artificial human body organs for medical purposes. A field of research completely different than the nature of Indian Poker cards. That's the first interesting thing about this girl!
Before the events of the arc, Kuriba Ryouko who was researching cyborg body parts volunteered to be the test subject of one of her experiments. Her body was split in two, and every missing parts of each body was replaced with machines. Turning her into two Cyborgs. Each Kuriba Ryouko was to live her life independently for a short while, then both of them will be merged into one again.
The experiment was a success! Not only both Ryoukos managed to live their lives without a problem, when the two were merged again, the original Ryouko managed to retain memories of both of them. She also got all the data she needed from such research, allowing her to get closer to her true goal.
There was one problem however, as without her knowledge the institution that experimented on her had another goal: The creation of "Soul." By making sure there are two of Ryouko, the scientists theorized that each one of them will have a separate Soul in her body and when the two are merged the other Soul will be free to put anywhere. The scientists used the mechanical parts for each Ryouko to create a Cyborg which will contain the other soul. They named her Doppelganger.
The doppelganger believes she's the real Ryouko, and she couldn't understand why the researchers won't let her out of the institution after the experiment was a success, but she decided to play along for a while.
Despite Ryouko concerns about the doppelganger and how dangerous she would be if the soul theory is correct, the scientists behind the project refused to listen and would only let the machine live her life thinking she's a human to gain data. When Ryouko told them that the doppelganger will eventually realize she's not a human, the researchers replied that they'll just get new data then.
Knowing she won't be able to do something about the doppelganger by herself, as well as that harming the cyborg could lead to a problem if her theory about the soul is correct, Ryouko deviced theorized a way to get a solution to the problem: Indian Poker cards.
She knew that spreading cards that allow people to share dreams would create a chaos on a large scale, but for her to find a way to stop doppelganger without harming her, she needed the knowledge these cards will provide. According to her, a solution will be statistically found in around two months.
Ryouko spends the next few weeks keeping herself away from the institutions in case the doppelganger realized that she is a copy and decided to eliminate the original. She spends that time alone trying to complete her cyborg body parts research to help her mother.
When she was a young, Ryouko was caught in an accident she only survived because she got a lung transplant from her mother. Her mother's body was already weak before that, and she worsened after it. Doctors predicted that even with medicines delaying the infection she already has a year or two to live.
Ryouko's desperation to find a cyborg parts suitable for her mother is what led her to volunteer in that experiment in the first place. She needed enough data for organs that her mother needs but she couldn't get the data in time through normal means. She was supposed to work full time on the data after her merge but the doppelganger situation was delaying her. Ryouko had to stay all night in her room to complete her research...
That is until she was found by Misaka Mikoto, Shokohou Misaki who told her off about creating the Indian Poker. They also told her that the doppelganger had escaped the research center, something she predicted to happen around that time. Ryouko told them about her plan to stop the machine and that waiting is the best course of action right then...
As things escalate further, Ryouko whose original plan to stop doppelganger have failed, quickly completes her research, then strives to help Mikoto solve the doppelganger problem. At the time of this writing, Dream Ranker's arc is currently going in this direction in the anime...
I Love Kuriba Ryouko's Character
Not only her design is amazing. She has one of the best balance of strengths/weaknesses in the series, she's very smart but physically weak. She's not so evil to let her doppelganger destroy the city, but isn't kind enough to stop spreading Indian Poker cards despite knowing all the problems they would cause. She believes that science needs sacrifice, but is reluctant to sacrifice anyone but herself to pursue her goals.
I think I loved her since she was first introduced because she's related to two things I love about the arc: Indian Poker as its creator, and Cyborgs as she volunteered not to become one, but two of them! That requires courage!
Everything about Kuriba Ryouko and the Dream Ranker's arc is amazingly written, and I can't help but consider her one of the central characters of railgun despite only appearing for a short time in this arc (at least at the time of this writing.)