Kaga is the second boss from 1943: The Battle of Midway, and the first aircraft carrier the players face in the game. It was named Kaku in the NES version.
Real World Information
Kaga (加賀) was an aircraft carrier of the Imperial Japanese Navy (IJN), the third to enter service, named after the former Kaga Province in present-day Ishikawa Prefecture. Originally intended to be one of two Tosa-class battleships, Kaga was converted under the terms of the Washington Naval Treaty to an aircraft carrier as the replacement for the battlecruiser Amagi, which had been damaged during the 1923 Great Kanto earthquake. Kaga was rebuilt in 1933–35, increasing her top speed, improving her exhaust systems, and adapting her flight decks to more modern, heavier aircraft.
For more information see the Wikipedia Article
In the Game
Kaga sets the standards for all aircraft carriers faced later in the game. It is only equipped with small guns which line it's edges, while the hull has two elevators from which enemy aircraft are launched to attack the player. Destroying the first elevator fast will reveal a hidden Holstein.