Short answers, pulled from the story.
Sakura Kasugano was created by character designer Akira "Akiman" Yasuda during the development of Street Fighter Alpha 2. He designed her as a deliberate departure from what he saw as the series' narrowing aesthetic, submitting her sailor fuku design despite expecting the project leaders to reject it. Additional artwork was contributed by Naoto "Bengus" Kuroshima.
Sakura Kasugano first appeared in Street Fighter Alpha 2 in 1996. She was introduced as a young Japanese fighter who had watched Ryu win the first World Warrior tournament and wanted him to train her.
Sakura idolizes Ryu and repeatedly seeks him out across multiple games to request training. Ryu consistently declines, telling her he still has much to learn himself. In Street Fighter V, her friend Karin Kanzuki sends Ryu to Sakura's house to spar with her.
Designer Akira "Akiman" Yasuda chose the sailor fuku because of its deep cultural resonance in Japan, stating that female high school students are "all-powerful in Japan" and "pretty much at the top of hierarchy" as character types. He also submitted the design knowing it would challenge colleagues who wanted only conventionally "cool" characters aligned with the aesthetic of films like The Matrix.
In Street Fighter V, Sakura received a complete redesign to reflect that she had finished school and was no longer a high school student. Her new default outfit was based on clothing worn by Japanese idols, and she wears a red headband as a mark of respect for Ryu. The original schoolgirl outfit remains available as an alternate costume.
Sakura appears in numerous crossover fighting games including the Marvel vs. Capcom and Capcom vs. SNK series, Street Fighter X Tekken, and Rival Schools: United by Fate. She is the lead character of the drama CD "Street Fighter Zero 2 Another Story" released by Victor Entertainment in 1996 and the central figure of the manga series Street Fighter: Sakura Ganbaru! by Masahiko Nakahira. A Sakura cosmetic outfit has also been added to Fortnite.