Actually, the video gives a bit of a skewed impression in that Miranda isn't the
love interest in the game. Rather, it's an option the player may pursue, but there are other potential partners, and accordingly one's relationship with Miranda may also be dominantly professional in tone and romance-free (as it was for me when I played the game). However, that's precisely what makes it so popular to upload the romance stuff to YouTube (due players who chose to go other ways being curious about it) and made this the easiest to find longish video sample of the character.
The character's personal story in the game is mostly about her family background and how it leads to her becoming involved with a terrorist organization the player character finds himself (or herself; one may play as a woman) making an uneasy alliance with, and how she tries to shield her sister from her father and going down a similar path as her's.
Do keep in mind these games are about 60 to 80 hours in length of which many hours are dialogue; there's room to show different sides of a character. I'll admit that for the lack of a better one that video was a fairly poor example though.