Suguha is the cousin and adoptive sister of Kirigaya Kazuto , but was raised along with him as his sister. In order to protect Kazuto from the wrath of their grandfather when he decided to quit kendo, Suguha became an accomplished kendo practitioner in his stead. Suguha has short black hair and dark grey eyes, almost blackish. Due to her kendo training, Suguha has strong and defined muscles that show up even when she makes small movements. Suguha's three sizes are  in the web serialization. In ALfheim Online, Suguha appears more like an adult than a child.
Confused about Kirito and Leafa.
Would kirito be in love with sugu if SAO incident never happened? - Quora
While Kazuto was trapped in Sword Art Online, Suguha wanted to know what Kazuto liked about virtual reality massive multiplayer online games, so she talked about it with Nagata Shinichi Recon — a boy from her class who had the most experience with the VR games — and bought a safer FullDive gear, the AmuSphere, to play ALfheim Online as a Sylph warrior. She likes in-game flight so much as to have earned the nickname "Speedaholic" from Recon. Suguha is cheery and confident, and much more social than Kazuto. Her character is strong, and she remained positive around Kazuto even after he started distancing himself from her after finding out they were not siblings and getting conscious about it. Suguha does not have much tolerance to the annoying or hostile people, and is quick to lose her temper; a good example was her punching Recon who had confessed to her and tried to kiss her. With all her physical and mental strengths, Suguha can still get lonely and fragile, especially around Kazuto.
In the real world, Suguha is the cousin and adoptive sister of series protagonist Kazuto Kirigaya also known as "Kirito" ; after he becomes trapped in the eponymous Sword Art Online video game in the series' first story arc, she starts playing a similar VR game, ALFheim Online , to better understand the world in which her brother is imprisoned. After her mother reveals to her that Kazuto Kirigaya is not her biological brother, Suguha starts to develop romantic feelings towards him but is later saddened after realizing his attachment to Asuna Yuuki. The character has received mostly positive critical reception, with praise centered on her character development and portrayal.
Suguha has been shown to be a very aggressively sporty and a very sarcastic person, possibly more than Kazuto. She doesn't even show gratitude when Kazuto goes out of his way to lend her a juice while she was choking on a muffin, and immediately starts mocking him as soon as she catches her breath. In addition to her jabs, Suguha is merciless when it comes to beating Kazuto in a kendo sparring match, to the point it can be viewed as domestic abuse.