Responses

I think its because Sae is a midfielder that Isagi was anguish. If you look on the current position, both Sae and Isagi holds the same foothold controlling their forwards— Shindou and Rin respectively. The thing is, when you go back to the time Isagi first play with “the demon” of bluelock, Shindou, he couldn't control or make a chemical reaction with him at all. And seeing Sae assisting Shindou easily, is a slap for Isagi.
All of this is just on my perspective and my opinion, it can be wrong— but I thought sharing this just to somehow enlighten you. (⌒▽⌒)
What’s wrong with Sae being a midfielder instead? I dont understand the anguish at all