Miho was a nasty bitch who conned and cheated people and, yet again, she fell into the "female character = love interest" box. Shinji could have done so much better than her (and, in fact, he has to cos she got killed off)
"THAT FEMALE CHARACTER IS DATING ANOTHER CHARACTER!THIS IS SEXIST!"
But you don't know what she's going to do.
Well let's see. Compare Miho to the other female Riders.
Faiz: bunch of girls used belts at least once. Didn't affect their characters much.
Miwa(?) / L'arc: A jerk for no reason. Why did she and Lance even exist? Glaive used the Cerberus card that Tennouji creates in the finale, so does that mean two copies were made? So she doubly fails.
Shuki: nothing spectacular.
Kiva: See Faiz
Decade: Kivaara is just part of the ridiculousness of Decade that makes no sense.
It is when that is their entire purpose in the story (like Nadeshiko.)