Yumi Ishiyama (voiced by Mirabelle Kirkland) is the oldest member of the group, being a year older than the others and is of Japanese descent. Branching from this, she owns a suit a samurai armor handed down the generations she treasures very much, as well as being seen with a Totoro plushie in her room. Although sometimes a bit short-tempered, Yumi is very clever and cares greatly about her friends. Yumi varies between reasonable and stubborn at various points; though willing to reveal the supercomputer to the police in order to prevent a nuclear explosion, it takes the destruction of most of Lyoko to convince her to let William Dunbar join the group. For the 1st and 2nd season she is 14-years-old and in 9th grade. In the 3rd season she is (surprisingly) still 14 but is in 10th grade. In season 4 she turns 15 and is still in the 10th grade.
Yumi's hair is half-long black, headed back and sides. In the first three seasons, she wears a black turtleneck, darker black jeans, and gray-black sneaker boots. Her hair is usually left hanging down. In the fourth season, her outfit is slightly lighter but still mainly black, consisting of a long-sleeved shirt with an extra large neck hole. Her pants cut off halfway between her ankles and knees.
In terms of character development, the two major themes revolving around Yumi and her relationship with Ulrich Stern and her parents' unstable marriage, the former of which is the central focus. Yumi's relationship with Ulrich remains platonic, though they almost kiss in "Routine" and decide to keep it a secret. Yumi decides to outwardly declare their relationship to be just friendship at the start of the third season, though the episode "Cold Sweat" and the series finale "Echoes" implies the two eventually explore their romantic relationship again.
The second season introduces Yumi's younger brother, Hiroki. He constantly teases Yumi about Ulrich. Most of the time, Yumi acts as if she can't stand Hiroki, but episodes such as "Lost at Sea" show that Yumi does care for him.
Yumi's outfit is modeled after those worn by geisha. Her outfit consists of a short-sleeved/hemmed kimono over red tights, black thigh-length boots, red eye liner, red blush on her cheeks, a white-painted face and neck, and some red on her lips to complete the look. She also has a yellow sash tied to her back. Her hair is bunched up into a small bun with three bangs left hanging. In the fourth season, Yumi gets a new, ninja-like outfit. It consists of form-fitting, pink and purple body suit with flowered shoulder pads, but retains the yellow sash from the previous one. Her hair is worn down instead of in a bun. The geisha makeup is less prevalent with the new outfit, though it's more apparent with her 2D model. However, she has two strange pill shaped objects above her eyebrows. These are used to start her telekinesis. Yumi uses the Overwing, a hovering scooter, for travel on Lyoko. She usually shares this vehicle with Aelita.
Yumi's main weapons on Lyoko are her dual metal fans or Tessen (Samurai metal fan) that function like boomerangs when thrown. In flight, they turn into fast-moving, saw-like discuses, and also serve as shields to block enemy laser fire. Her fans now appear and disappear at her will (as long as they are in her hands). In the first season she only has one large fan, but in season 2 onward she has two slightly smaller ones. She also has telekinetic powers, though they are difficult to use and exhaust her quickly.