For those that can't read Japanese and are confused with the plot, a quick summary:
In minarai's interpretation of the kancolle world, ship-girls are normal girls given and imprinted the soul of the warships, essentially they are hosts for spirits.
Once imprinted the host's memory is suppressed, and the warship's spirit possess the body. The host's memory comes back after the spirit leaves the body, usually when they sink. The spirit itself can be copied over and over (explaining why there is multiple shipgirls of the same type).
Due to the nature of the possession it is taxing on the host's body, and once the spirit leaves the host that host cannot be a ship-girl ever again.
In this comic the first Kitakami sank, lost her memory + spirit, and the host girl left the military; for obvious reasons Ooi is traumatized. After a while a Kitakami replacement came in with a different host and new memory (spirit), and Ooi decides to let it go (but not before mentioning that the first Kitakami will always be the "best girl.") And the the second Kitakami comes in and vows to be better than the last "me."