While I think we can all agree that all the gals in bakarina get pretty routinely sidelined I think Mary might have it the worst.
Maria is the clear front runner and, while I felt like Sophia's depiction in the manga/second season of the anime was pretty lackluster, she at least had a really strong presence in season 1 as her own character separate from Nicole. Mary just hasn't had a chance to shine in the story and it's such a shame. She's literally gotten no focus since they became teenagers and her personality is usually boiled down to "I fight with the boys to keep them away from Katarina".
It just feels wrong that she doesn't try to confess, and seems content to play damage control rather than be proactive about wooing Katarina.