is a Japanese actress and singer. She has voiced many different types of characters, from young girls to adult women, as well as boys and animals. Her roles include leads such as Ryuko Matoi in Kill la Kill, Charlotte E. Yeager in Strike Witches series, Kallen Stadtfeld in Code Geass, Anemone in Eureka Seven, Irina Tachibana in ', Nadja Applefield in Ashita no Nadja, Leonmitchelli Galette Des Rois in Dog Days, Shizuri Mugino in A Certain Scientific Railgun series, Yumi Hoshino in KimiKiss: Pure Rouge, Takuto Hasegawa in Magician's Academy, Hibiki Hojo in Suite PreCure, Nanaka Yatsushiro in Myself ; Yourself, Ersha in Cross Ange, Himari Noihara in Omamori Himari, Sailor Jupiter in Sailor Moon Crystal, Mikumo Guynemer in Macross Delta, Nodoka Haramura in Saki, Tenma Tsukamoto in School Rumble, Holo in Spice and Wolf, Ibuki Mioda from the Danganronpa series, and Yang Xiao Long in both RWBY (Japanese dub) and '. In video games, she voiced notable characters like Agnès Oblige in Bravely Default, Mai Shiranui since KOF Sky Stage, Mist in Rune Factory, Rimurisu in Tears to Tiara, Rinka in the Atelier series, Miyabi in Zenless Zone Zero (Japanese Dub), Rosa Ushiromiya in ', Tsuruhime in Sengoku Basara, Yukiko Amagi in Persona 4, Captain/Major Claire Rieveldt and Musse Egret in The Legend of Heroes: Trails of Cold Steel series, Beidou (Hokuto in Japanese) in Genshin Impact (Japanese Dub), and Zima and Projekt Red in Arknights. She also is known for her dubbing role for Thailand films which starred Yanin Vismitananda. In addition, she also voiced in the live action anime film Shimajiro and the Rainbow Oasis.
At the age of 12, Koshimizu joined Theater Company Wakakusa. In the second year of junior high school, she was over 160 cm tall and was troubled by the fact that she could not get roles as a child actor. Influenced by her senior in the troupe, Mayumi Iizuka, she began to aspire to become a voice actor. In 2003, she passed an audition for Ashita no Nadja and made her voice acting debut as the lead Nadja Applefield. She continued her career with roles in School Rumble (as Tenma Tsukamoto), Onegai My Melody (as Miki Sakurazuka), and Eureka Seven (as Anemone).
In 2007, she won the 1st Seiyu Awards for Best Actress in a supporting role for her role as Kallen Stadtfeld in Code Geass, and in 2011, she starred in Suite PreCure (Hibiki Hojo/Cure Melody) for the second time since her debut in the 8:30 - 9:00 Sunday slot produced by Asahi Broadcasting Corporation. She also visited Anime Expo as a guest of honor to promote Kill la Kill as Ryuko Matoi.
On September 30, 2011, Koshimizu left Production Baobab, where she had belonged for four years, and joined Axlone on October 1, 2011. On February 28, 2018, she left Axlone and announced that she would be working as a freelancer. On January 7, 2022, she announced that she will be under her own agency, Office Restart.