Seriously how the fuck is this guy not a hentai artist, these character expressions are so sus
Edit: in the years since this series started, that has changed.
Go back to the start of the series, and really think about how someone ends up living the way he did. And then look at the stuff we got in the last few chapters. You'll start getting the picture
Look at the earring and necklace closer, they're meant to give you a fun hint as to why Saito is next in line and not his dad. Also that's not a thick lip, it's an open mouth.
I'm guessing whatever divine protection from the gods that gets applied to the heroes is what's fueling the summoning. Since Tsukuru got fuck all from this world and instead got something hidden from Amaterasu (called Susanoo her brother) he wasn't on the radar as anything useful.