You can't challenge a god, about love. He'll get the girl sometimes, but the percentage will increase when he had assimilated his powers with her anyway. It's not like imprinting, but they're simply made for each other. No matter how the other boy is present from the beginning, she was already connected to Suijin because of that water-gate thingy. If not for that, she'd never meet the boy as her early companion. Her bond with Suijin trespasses worlds, from the start.