Mizumachi on Kakei
When they first met, Mizumachi was drawn in by Kakei's determination. On the other end, Kakei understood and appreciated Mizumachi's talents, and was fully willing to work alongside the latter. Having had the swimming team crush his dreams of working with them to nation-wide top, this was highly significant for Mizumachi.
From then on, Mizumachi clearly admires Kakei. He muses about Kakei with almost puppyish infatuation and is so enthusiastic about letting other people know of Kakei's awesomeness that it's easy to deduce that he sees Kakei at a higher level than his other friends. In addition to that admiration, is the absolute trust he puts in Kakei. In his simple statements in volume 13, chapter 115,
"I joined American Football, because of what you said."
"You told me didn't you? We'd go to the top."
it's almost painful just how much faith he has in Kakei.
At the same time, Kakei's the only one who almost never questions Mizumachi's actions. He lectures, he blushes, he looks like he wants to bury himself under the bleachers, but he rarely questions. When Mizumachi strips - his captain Kobanzame always asks, "So, why are you taking your clothes off?" after the millionth time Mizumachi has done it, but Kakei doesn't blink an eye. When Mizumachi randomly joined a skirmish - the Hiroshi twins yelped "Why's Mizumachi joining the fight!?" while Kakei wordlessly yanked Mizumachi off by the back of his collar. It might be because Kakei has grown so accustomed to Mizumachi's quirks, but Mizumachi probably appreciates this more than even he himself is aware of.
Of course, the manga's also littered with small incidents that hint how close Mizumachi is to Kakei. One such incident can be found in volume 13 chapter 111. Sena had arrived with the intention to spy on Poseidon under the pretense that he was delivering their new cleats, but Kakei caught him red handed. Sena then told Kakei that he was just a secretary for the Devilbats, so Kakei broke out the tea and cookies as a form of apology. Mizumachi stepped into the clubroom.
"Huh? Kakei's serving tea! What's the occasion? I thought you really hated Deimon?"
He'd have to know Kakei well enough to judge that Kakei seldom served tea unless there was a need to, and Kakei must have said more about "fake Eyeshield 21" to Mizumachi for the swimmer to have questioned Kakei's liking for Deimon.
BFF? Oh yes.
Then, is he gay for Kakei?
Yes, and no.
Mizumachi's far too naive to actually understand the idea of a romantic relationship in the strictest sense. My take on it, is that Mizumachi does indeed have strong feelings for Kakei, strong enough for him to attach to Kakei's hip despite the millions of other friends he has in school. But, he doesn't recognise these feelings as romantic at all.
To put it very simply, all Mizumachi really wants is to be with Kakei. Till death do them part.
If Kakei wants to be friends, awesome! If Kakei wants to be more than friends, sure why not! Chances are, Mizumachi won't see a difference between being best friends and lovers, other than the fact that there might be more touching in the latter category. And he likes touching.
So, unless someone prompts Mizumachi to poke Kakei about their relationship, it'd really be all up to Kakei to initiate. Which, probably won't happen in the near future 'cause... Kakei.