Do You Want a CO-STAR Again?

Sure she and Jacob were connected to the story but if you are going to be fair Mordin, Legion and Tali had a much bigger role thanks to their storylines (Genophage and QuarianXGeth conflict).

So i don't agree with you OP.

Mordin wasn't required since Padok Wiks takes his place while still able to cure the genophage. Legion and Tali are only required for peace. Tali didn't have to be a squadmate for that.

I thought Miranda was the protagonist.^^
Shepard the beautiful eye catcher.

Did she though? Every crew member got a lot of screen time. I'd bet she would have the most only because she's one of the two starting members as well as a significant relationship with Cerberus, but even then her presence probably only wins out by a small margin. I don't think you could definitively chalk her up as the "co-star" unless you deliberately made it that way. It's not like every conversation you have with TIM goes through her. I wouldn't actually mind it if they did though. If it makes sense plot-wise why not?

Liara is the closest thing Mass Effect has to co-star considering how integral she is to each game's plot. I don't mind that dynamic in theory, but in practice Liara's character progression was a bit awkward. If we got a better sense of how she became a badass information broker, I'm sure that would have been cleared up. It would also help if she was less of a Mary Sue. If I'm going to be opposite a character for a long time, I'd like for them to have more interesting (even unsavory) facets about them (e.g. Mordin).

However, if we want to extend the definition of co-star out to the antagonist, then my answer is definitely. The central plot of nearly every BioWare game has lived or died on the shoulders of its antagonist, mainly because BioWare seem incapable of making a coherent plot that's inherently interesting. Since I have little faith that BioWare will just miraculously start writing deep and coordinated plot-lines after all these years, I can at least hope that the villain will get enough attention to distract from the inconsistencies. I'm fairly confident that the main narrative arcs of Mass Effect 2-3 and DA:I would have been as well regarded as Mass Effect 1's if their antagonists were deeper characters with enough screen time to develop.

Loved Miranda in ME2 and she accompanied my Shep everywhere and was his significant other.
I still don't think i'd describe her as a co-star. Jacob and her had a more active role in between missions content because of their role in the story as part of link to cerberus. I don't mind if there's similar justification for more active involvement this time round either.

Neither is Miranda to be fair...

Liara is pretty much the only one to fit that bill. 

After Mars she isn't. Sure she's on Thessia, but shouldn't of been required

Both Legion and Tali came way too late in the game.  Simply put they didn't get enough screen time.  Matter of fact Quarians always seemed like a background race.

 I am just pointing out even then have bigger roles in the story than Miranda.

Like some people already said Liara would be a much better option to the role considering all her story in the trilogy, but even then i would not classify her as such.

Mordin I get. More for  dealing with the seeker swarms than anything else. The other two? If they're that important, they should've been required. Since they aren't, they aren't important, at least when it comes to ME2

Whew! I thought you were going to say Diana Allers and I was going to reply "NO!"

I still don't understand why Shep couldn't punch her.

BTW, let me fix your OP for you:

"Think Miranda from ME2. It was clear right from the opening bell that your video game girlfriend was going to have a bigger role in marketing the game and promotional collateral (posters, web ads, whatnot) than any other sidekick."

And if you were a producer and cast Yvonne Strahovski, you'd do the same thing, since she was a hot property at the time. What ever happened to her?

A discussion with quality arguments? BSN is officially dead...