Yaku Yakuzen (薬やく膳ぜん ヤク, Yakuzen Yaku?), is a character in The 100 Girlfriends Who Really, Really, Really, Really, Really Love You. She is Kusuri's grandmother, Rentarou's 16th girlfriend, and a retired military medic. Yaku is 89 years old, but still has the physical appearance of an young girl due to ingesting a prototype of the immortality drug Kusuri gave her. She is a widow, and met her first husband Isahito Yakuzen in a warzone where she was serving as a combat medic. Like the rest of the Yakuzen family, Yaku is knowledgeable about chemistry and pharmacology (especially medicinal plants). Due to her advanced age, Yaku acts more maturely towards Rentaro and the other members of the Rentaro Family, and ends her sentences with an antiquated version of her granddaughter's verbal tic nanoja (なのじゃ).