Our Alucard/Walter knock-off is the usual over-powered albeit not so chivalrous MC.
The change in character was a bit sudden but all in all this one is above average among it's many peers.
They're going for a swim so of course they'd be wearing swim wear, what's wrong with that? She thankfully wasn't drawn in some cringy "ecchi" pose either.
At that age it'd wouldn't even be out of the ordinary for the girls to go swimming topless not despite but BECAUSE they're children.