@LeannaCR
That kinda was the point, he was essentially unpersoned by his entire school for being Kenji's scapegoat for stealing the badge. Then when Fukubei started his Friend cult, he reached out to the two people that almost no one could suspect: Sadakiyo, who was almost entirely forgotten, and Katsumata who "died". That was one of the reasons why Donkey was killed, he saw all three of them in the science lab.
@gr8g
Katsumata was the one who wanted to dissect the carp but "died", yes. But no one actually died during Fukubei's "miracle in the science lab", except for him maybe almost killing himself. If I had to make a guess, the ESP that Fukubei, and thus Kanna, had were the results of him getting people to try to make him more "special", since he only had the gift of manipulation and slight of hand as a child (maybe, it's still unclear).
The real laser gun was probably also a fake at the time, like the original robot. And Kanna's mom was probably aware, but didn't really care since she was more interested in making the supervirus. There's probably more in the "director's cut" of the manga, in addition to the altered ending, but that hasn't been translated/scanned/released in English yet.