The I-201 is an I-201-class submarine operated by Higashi-Maizuru (Toumai) Boys' Marine High School as a training vessel. It is the only submarine the Harekaze encountered throughout Hai-Furi.
The I-201 is the lead vessel of its class of fast submarines. Electric welding and lightweight diesel engines significantly improved its speed over its predecessors, which were only able to clock in at no more than 10 knots. Even though the I-201 never travels at top speed (for safety reasons), it's still fast by submarine standards, especially when submerged.
The submarine carries ten Type 95 torpedoes, a smaller variant of the successful Type 93 "Long Lance" torpedo carried by the Harekaze. Despite having shorter range and a reduced dimension compared to the Type 93, the Type 95 packed a much more powerful warhead than any submarine torpedo fielded at the time. A smaller torpedo means it's also faster underwater.
As the Harekaze planned to sail back to Yokosuka to explain the skirmish with the Sarushima and Admiral Graf Spee, sonar operator Kaede Marikouji detected the I-201 pursuing the ship at 3,000 meters away. Attempts to send a message via Morse code through active sonar were mistakenly perceived by the I-201 as hostile action.
Suddenly, the Harekaze was thrown into an anti-submarine battle with the I-201. The submarine fired nine of its ten torpedoes, only scoring close calls. The Harekaze deployed a paravane and let it loiter underwater to search for sounds of metal contact. The paravane also inadvertently fouled up one of the I-201's propellers with its wire, allowing the depth charge to force it to surface.
The Harekaze made its escape into the night. An instructor ship from Toumai rescued all crew members of the I-201.