Haruko Momoi, born December 14, 1977, in Tokyo, is a Japanese singer, songwriter, and voice actress professionally known by the nickname Halko, a name she coined for herself inspired by the HAL 9000 computer from the film 2001: A Space Odyssey. Her career began in the late 1990s with street performances in Tokyo's Harajuku and Akihabara districts. Her activities and online writing led to a job as a columnist for Weekly ASCII magazine while she was still a student. She made her formal debut in 2000 with the single "Mail Me," followed by her voice acting debut in 2001.

Momoi's voice acting career began with her role as Komugi Nakahara in The SoulTaker and its related OVA series Nurse Witch Komugi, establishing an early association with energetic and magical girl characters. In 2002, she formed the musical duo UNDER17 with Masaya Koike, performing theme songs for anime and video games until the group's dissolution in 2004. Following this, she continued as a solo artist and signed with the Avex label in 2006 as a composer and singer.

Throughout her career, Momoi has portrayed a wide range of characters across anime, OVA, and video game projects. Her notable anime roles include San Seto in My Bride is a Mermaid, also known as Seto no Hanayome, which aired in 2007. She is widely recognized for her role as Faris NyanNyan, also known as Rumiho Akiha, in the 2011 science fiction series Steins;Gate, a character she later reprised in Steins;Gate 0 in 2018. Her filmography also includes Mii in Popotan, Mai Nonomiya in Lovedol ~Lovely Idol~, Utamaru in the Da Capo series, and Tsukune in the OVA Majokko Tsukune-chan. In 2014, she voiced the original Wonder Momo in the anime adaptation Wonder Momo. Her work extends significantly to video games, with prominent roles such as Anise Tatlin in Tales of the Abyss, a part she also played in related games, and various roles in the Steins;Gate game series.

Momoi's career is marked by her deep connection to Akihabara's otaku culture, earning her the informal title of "Original Queen of Akihabara". In 2007, she published an autobiography titled Akihaba-LOVE, chronicling her experiences growing up in the district and her career development. She is also the producer of the all-female pop group Afilia Saga. Beyond her work in Japan, Momoi has performed at numerous international anime conventions across the United States, Europe, and other regions.