After starting my latest rewatch of Gachiakuta, I noticed something, Regto isn’t as pale as other Spherites and after reading the manga, I think I might be onto something that I just can’t prove.  
What if Regto is one of the other “Apostles” such as Rudo’s father. Rudo’s dad was the “Undertaker”, the first “Spellcaster” was Macaca Icol and we know nothing about the other two. We know that one of them was called “The Nameless Recorder”, and in the beginning of the first episode we can see Regto with a book that has the Watchman series logo and he was writing something in it, and from the manga (specifically the latest chapters) what I understood is that the four apostles were able to the sphere and return to the ground. My point is, what if Regto was that “Nameless Recorder”, who was probably recording the history of the ground and sphere (or at least that’s what I think), and he went up to the sphere with Rudo’s dad and was entrusted to raise Rudo on the sphere and record their history in Rudo’s dad’s journal for whatever reason, that might explain how Regto had the R3 gloves (Rudo’s gloves). And because Regto was an apostle, he probably possessed some power, or knew something about the Gods, that’s why he got killed.  
I thought about sharing this theory, and I am sorry about not being able to make it more clear, English is my second language and I’m not quite good. I tried my best to make it understandable, and I wanted to share my theory

MANGA SPOILERS AHEAD‼️ I’ve got a theory about Regto