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|Japanese voice actor||None|
Yotsuba's grandmother, usually called "Grandma" (ばーちゃん bā-chan) by her granddaughter, is the mother of Yousuke Koiwai and grandmother of Yotsuba Koiwai. She lives with her husband in the country, and allowed Yousuke and Yotsuba to stay at her house before they moved to their own place. She first visited her son and granddaughter's new home when she was in the area for a concert with a friend called Hiramatsu. She comments on her husband being quite incapable, as he wouldn't be able to cook, clean, take out the garbage or even bathe without her help (he was formerly hospitalized according to Yousuke). Yotsuba's grandmother is originally from the Kansai region of Japan, which still reflects in her accent to a degree according to Ena.
Appearance and personalityYotsuba's grandmother greatly resembles her son in looks, being tall and thin with square glasses and short white hair. She is obsessed with order and cleanliness, to the point of scolding people for not tidying up their shoes after going outside, and gives unsolicited advice to Koiwai about buying a car, buying a house, and planting a garden. Though usually serious and somewhat strict, Yotsuba's grandmother has a deadpan sense of humour similar to that of her son, and is open to playing along with Yotsuba's shenanigans.
She is also quite generous, giving away kiwis and pears as well as colouring pencils, a pencil sharpener and personalized aprons for cleaning and cooking to her granddaughter. Additionally, she is keen to share her knowledge on various subjects, as she willingly taught Yotsuba to bake, clean and how to make origami.
Yotsuba Koiwai is her granddaughter, and they have a close relationship. She indulges Yotsuba in sharing her thoughts with her and showing her the drawings she made. Yotsuba's grandmother has a firm hand in making sure her granddaughter learns to keep the house and yard clean, changing her negative attitude to doing chores, as well as scolding her when she does something irresponsible. Yotsuba thinks her face is scary-looking, but tells Fuuka she is not as angry as she seems. Though Yotsuba claims she loves her grandmother because she "gives her things", she got very upset when her grandmother's stay at the Koiwai residence was over, even hiding her luggage in an attempt to stop her from leaving. Her grandmother promised she would come back one day, and to let Yotsuba and her father visit her house as well. She also told Yousuke she wanted to make video calls to Yotsuba with her iPhone.
She is the mother of Yousuke Koiwai, and upon starting her temporary stay at his house she immediately expresses a desire to clean things up and even wax the floors, providing a sharp contrast to his laid-back, slacker persona. They appear to get along fairly well, though Yousuke is sometimes embarrassed by her bouts of eccentric behaviour, such as always baking more than required, or attempting to impress him with her new iPhone. Unlike him, she has little to no knowledge on how computers work, or technology in general.
Little is known about her relationship with Yanda, though it is shown to be mostly friendly. He appeared pleasantly surprised that she was still alive when he encountered her at Koiwai's, and she even said he was welcome to visit her house anytime. Though, like Koiwai and Jumbo, she enjoys teasing him, claiming that his hair appeared to be thinning much to his dismay. She also commented that Yanda would be a bother if he went to her house, despite asking him to herself.
Yotsuba's grandmother had a brief encounter with Fuuka outside Yotsuba's house, catching her at an embarrassing moment when she had meowed at a bag she mistakenly believed to be a cat. Yotsuba's grandmother thanked Fuuka for being around and playing with Yotsuba.
Ena joined Yotsuba and her grandmother in making origami, and her grandmother's display of skill greatly impressed Ena. She also remarked on Yotsuba's grandmother's accent, since she had a teacher herself who spoke in Kansai dialect.