[Kiwami]

It is him, and he's probably a founding member of Faust as well.

He founded Faust alongside Katsuragi with the intention to utilize the Martian technology, they activated Pandora's box once but then needed the Fullbottles to continue any further. His daughter had the bracelet and the power to purify them.

At this point I'll imagine something went wrong, Gentoku Himuro took over, wanted to use everything they had to wage war and mass-produce the Rider system, and he was willing to abuse Misora to purify the bottles at the expense of her health.

This among other things may have caused a rift between the Faust members. The owner then helped Katsuragi to drop out: Katsuragi faked his death and became Sento Kiryu (The owner, as Blood Stalk, switched the faces of Taro Satou and Katsuragi) and then abducted him to expose him to Nebula Gas. That way his memories were wiped (but he knew his research was save, hidden by his mother and Blood Stalk would help retrieve it) and he was given the ability to become Build.

The owner picked up the amnesiac Sento, handed him the technology Katsuragi gave him and had his daughter purify the bottles. To Night Rogue he just argued that he had a way to create the Fullbottles easily: They created Smash, let them loose, Build would act against them, capture their essence and Misora would purify them out of her own will.

But we know that Stalk goes out of his way to help our gang, against the will of Night Rogue, so he obviously has some goal in all of this.

The clues are too obvious, it must be him. INB4 they pull something completely random out of their hats