We all thought Shepard was dating either Liara, Ashley, Tali, Jack, or Miranda. But confirmed from Jack, it turns out that Shepard's been dating an Alien from the Andromeda Galaxy. That can also explain the Andromeda Iniative, finding a new home, saving the various races from the Reapers, and finding what planet she came from.

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You're so handsome Commander~
Commander Shepard: We'll bang, okay?