We can tell this by listening to the music that is heard when Nine takes out his earbuds near the beginning of the first episode.Say what you want about about Zankyou no Terror as a show.
ZANKYOU NO TERROR ENDING SERIES
The Icelandic music Nine is seen listening to throughout the series is the same music that's playing during the ending scene, in which him and Twelve die.His associated colour from the Icelandic flag is blue.His death was symbolic as he was the brains of the operation. His death was from the headaches caused by the experimentation.It could be that because he couldn't save her back then, he felt powerless that he couldn't save her in the end. He has been seen to hate Five throughout the series, but at her death, he shouts out her name in surprise.As Sphinx Number 1, he wears a blue mask.However, this may be because he doesn't want her involved as he doesn't want her to be hurt. He is seen to be less caring towards Lisa, unlike Twelve.Another example is in Episode 6 and 7, he is able to visualize the chess board and where all the pieces were in order to figure out where Five hid the bomb. An example would be when he is telling Lisa directions to place the bomb (even telling her the number of steps) in Episode 1. He is capable of visualizing things as if they were right in front of him.In his alias, 九重 新 Kokonoe Arata, 九 is the kanji for 9.The romanji for his name is "nain" in order to sound like the English pronunciation, "nine".Nine still has nightmares about that day, as he was unable to save Five. While Nine, Twelve, and Five were trying to escape, Five decides to stay behind. It is heavily implied that a fire broke out, but it is later reveal in episode 9 that Nine and Twelve started it to escape. When they were younger, Nine and Twelve were part of a project called The Athena Plan whose goal was to educate gifted children. Twelve has described Nine as a sadistic but likewise a nice person. His emotional instability is exposed further when he is under great stress, as exemplified in episode 5 when he has a panic attack upon realizing that Five was onto him and his plans. Ever since, he has had recurring nightmares of the incident.
While escaping from what appears to be a fire that broke out at the institution, he underwent a traumatic experience of leaving Five behind as he was unable to save her. He was raised in a children's institution (little information is known about the facility) with Twelve, whom he was closest to, and Five. He has a knack for devising ingenious criminal schemes while deftly covering up his tracks with his exceptional computing and hacking skills. In his street clothes, which he is seen wearing throughout most of the series, he wears a black, long-sleeved shirt with rolled up sleeves and gray jeans. In his school clothes, which he is seen wearing on the cover and in episode 1, he wears black jeans with a brown belt and a white short-sleeved button-up with a green symbol on the right breast, presumably the school outfit for the men. He seems to favor dark colors, as he tends to wear dark clothing. He is supposedly quite handsome, since several girls, as seen in episode 1, asked for his number. During most of the series, he is also seen wearing a pair of glasses with dark blue frame, which he loses near the end of the show. His eye color is inconsistent and often varies depending on the light or surroundings, in most cases usually grey or black.
His black hair has been cut short with bangs reaching his eye level.
Nine is a tall young man of average build.