This is what I think the writers were aiming for with the love quadrangle:
1. AmataXMikono is the main pairing, and we'll establish that by getting them off to a fast start (which, to their credit, they did).
2. We'll have rivals on both sides (a male rival for Amata, a female rival for Mikono) to stretch out the romance conflict/drama of the anime narrative. By making it a "two-front war", this will effectively "kill time" until Amata and Mikono have their big, heartwarming romantic moment in the last episode or two. This will be the ultimate romance climax experience, as long-suffering and patient AmataXMikono shippers (which meant Aquarion EVOL fans
in general, Kawamori was probably hoping) finally get to see the two come together in a clear-cut and powerful way.
3. The male rival - Kagura - will be the main "front-line" antagonist of the show, and will play an important role in most of our action scenes.
4. The female rival - Zessica - will be the main female "action star". Like Kagura, she'll play an important role in action scenes, only from the protagonist side.
5. 3 and 4 ensures that Kagura and Zessica maintain plot relevance above and beyond romance conflict alone.
6. 5 is important since we intend to use Kagura and Zessica in convenient ways to build up the drama of the show, as well as contribute to Amata/Mikono's character development.
Here's the sad truth, I think - I don't think that Kagura and Zessica were ever important
in and of themselves from the writing staff's perspective. In other words, they were what I could call "complimentary characters" - There to serve whatever role the true main characters and the plot required of them.
This is where I think the writing staff goofed. Zessica and Kagura were simply intended to be the romance equivalent of Team Rocket - There simply to ensure conflict, and ultimately to "put over" Amata Ketchum and his dear MikoChu.
Amata and Mikono being less popular than Kagura and Zessica (respectively) was a totally unforeseen and unplanned development, I think. That's partly why Kawamori is so bothered by it, I think - It throws a mighty wrench into his plans.
It's also why I'm not hopeful about Zessica and Kagura's final fate. They've served their purposes, now they're probably going to die or get absorbed...