I-401 is a "Fog" submarine in possession of Gunzou Chihaya. Her Mental Model named Iona(イオナ) is the second protagonist of the series.
She is a reincarnation of a Sen-toku class WWII Japanese submarine, upgraded with "Fog" technology. Originally I-401, like her two 'sisters' I-400 and I-402, was a surveillance submarine within the "Fleet of Fog"; but she was later re-equipped as an attack submarine. She has therefore lost most of her surveillance equipment other than the basic sensors equipped in regular ships in exchange for more offensive weaponry.