Yumi has apricot skin, blue eyes and short light purple hair that she styles into a messy shoulder-length bob. She dresses in a rock style with a studded choker and two handcuffs, a purple T-shirt with a skull on it, a black miniskirt, purple shorts and black/purple punk boots. She often shows her highlighted fang in official arts and a pilot episode, while rarely does in main episodes.
For bedtime, Yumi wears a black or a white sleeved nightgown with a skull in the middle.
For swimwear, Yumi first wears a green camouflage bikini in the episode "Robo-Pop," but in "Surf's Up" and until the rest of the series, Yumi wears a purple string bikini with part a skull on the bikini top and the other part on the bikini bottom.
For formal wear in "Movie Madness", Yumi wears a glamorous black sequin dress with the same studded choker from her usual outfit and wears black opera gloves.
For ninja wear, Yumi wears a black sleeveless ninja outfit and wears a black balaclava.
As a magician assistant, she wore a pink headband, a golden necklace, a pink tube top, a pink miniskirt and gold pumps.
In "Secret Origin" and the flashback of "Super Zero", she wore a black long-sleeved shirt, a white flounced skirt, white socks and black Mary Janes shoes.
In the pilot, she had her hair being more purplish-toned and lighter, a dark magenta shirt with the skull having a green outline, slightly tan skin, freckles, a greyish-blue glove (on her right hand), spiked handcuff (on her left hand) and spiked choker, pastel bluish-purple socks and boots with shades of purple. She also had square-framed sunglasses on her hair in some shots.
Yumi's demeanor is not just like Ami's. She is sarcastic, cynical, vain, sassy, stubborn, and gained with self-importance. She loves things like ninjas and anything else rough and tough. She rarely signs in any excitement as "she is too cool" to show it. For some reasons, she comes off as rude and insults others she sees as "not cool".
Despite being similar age group with and more serious than Ami, Yumi is also more impetuous than Ami and rushes headlong into a decision before thinking. However, Yumi has a mature side, too, and for some reasons with Ami's childish obsessions. She also has a big heart and is willing to help while she can.
In the pilot, she was depicted being more calmer instead.
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- Like Ami, Yumi is not voiced by her real-life counterpart in the Japanese dub despite being modeled after her.
- Yumi is a only main character to have one fang.
- Yumi made a cameo appearance in the OK K.O.! Let's Be Heroes special Crossover Nexus.
- Yumi Yoshimura is voiced by Grey DeLisle, who is best known for Wubbzy from Wow Wow Wubbzy!, Frida Suãrez from El Tigre: The Adventures of Manny Rivera, Frankie from Foster's Home for Imaginary Friends, Sam from Danny Phantom, and Lola, Lana, and Lily Loud from The Loud House.
- Like her friend Ami, Yumi has her own motorized scooter in some episodes. According to puffycrew.blogspot.com, a former developers' blog, their motorized scooters were influenced by real-life Puffy AmiYumi's music video for Wild Girls on Circuit that includes retro-styled Yamaha Vino scooters.
- While her and Ami's accurate age is never revealed in episodes, some episodes like "Team Teen" and "Ninjcompoop" imply that the duo are teenagers as animated characters. Their ability of driving their own scooters and tour bus could be a clue to guess they are about 14 or 15.
- Fans of the show usually consider Yumi as younger counterpart of the animated duo than Ami. As real-life counterparts, Yumi Yoshimura(born in Jan 30th, 1975) is about two years younger than Ami Onuki (born in Sep 18th, 1973).