A Sapporo-based musical ensemble doujin group best known for remixes of music from the Touhou series.

Their song "魔理沙は大変なものを盗んでいきました", known in English-language fan circles and beyond as "Marisa Stole the Precious Thing" or "Marisa Stole Something Very Important", has, through its popular and memetic promotional video (post #82943), brought Touhou and the Alice Margatroid/Kirisame Marisa pairing to the attention of a huge number of people.

See also

• Official website: http://www.iosysos.com/
• Touhou Wiki on IOSYS: http://en.touhouwiki.net/wiki/IOSYS