^ While I agree that within what is shown in the show, Mary has about the least amount of chemistry, I think it's because unlike the others (except Anne) who are still developing their feelings for Bakarina and coming to understand those feeling (which is the heart throbbing phase that has the most potential to show chemistry), Mary had finished that phase very early. Everything Mary does afterward are sticking close to Bakarina, underhandedly indulging in a few moments of overflowed thirst (that episode where the harem got sucked into the magical book, Mary was excluded because then the show's rating would surely skyrocket), and thinking up plans to sway Bakarina toward her side (because Mary loves Bakarina and respects her choices), not exactly the right material to show chemistry.
The author also wanted to steer the story toward Maria, who strayed into Bakarina's well of gravity 7 years later than other and yet still the MC of the game.
And most importantly, Bakarina is Bakarina, denser than a blackhole and too concerned about her doom flags to even notice and examine her own feelings in respond to her harem's presence, much less the harem members' feelings for her. One sided chemistry is hard.