Yeah, i've tried a fair few times to write out how much worse she is than Akira but i always get lost in the words (trying to describe nuance using the same words leading to infinite regressions of explanation), last time i ended up settling on a rather empty summary of 'she's in a league of her own' but that felt pretty weak compared to how i feel about her.
See, they're both groomers, both are shit people, but Akira does really seem to be like a far lesser form of it, his main targets are the vulnerable midteens, easily manipulated to get into bed but also somewhat grown enough to be also somewhat making that decision for themselves. It's still pretty fucking gross and i'm not going to argue it's in any way okay, but it feel far more... common... or within the meat of a standard distribution. You're always going to have folks who prefer the barely legal/almost legal type so as gross as it is it almost seems like Akira is just your common, run of the mill sex addicted pervert.
Sekai on the other hand was a full blown, dyed in the wool groomer, targeting a literal child, warping his mind, fostering dependency, manipulating the concept of 'ok' behaviour, using him as a toy. every peek into her personality through Akira's busted flashbacks paints the very worst picture, the kind the makes your stomach turn over as you realise wtf those creepy ass words mean, the entire books behind each individual sentence... this is a kid who needs rescuing from a monster.