Undefined_N on Reddit brings up a great point: All seven shipgirls here were category winners in the Japanese Twitter poll ahead of the 4th Anniversary (3rd Anniversary for English version):
The one you want to spend the rest of your life with (Belfast)
The one you want to have as a elder sister (Atago)
The one you want to have as a little sister (Unicorn)
The one you want to have looking after you as an senior family member (Friedrich der Große)
The one you want to have as your boss (Vittorio Veneto)
The one you want to have as a classmate (Bremerton)