Like i say on the FAQ you can upgrade the girl you want in the menu.
N to R = 2 duplicate N
R to SR = 2 duplicate R + random card R
SR to SSR = 1 duplicate SR + random card SR
SSR to UR = 1 duplicate SSR + random card SSR
It's same for the girl with ( + ) a side of the name
PS : If you're girl are an Main girl and not a Generic, you need to put another Main girl in the random slot !
For an Generic girl you can put whatever you want
For loocking if the character is an main girl and if you play in R-18, just look on the H-scene zone and all girl in it are Main Girl
Or simply put her as adjudant (or secretary in Kancolle term) then poke her in the main menu. if the expression change, it's an main girl