In the Light Novel Volume 8, it is revealed that Momiji is actually the daughter of Hakurou.
At first conceited believing that Rimuru was just a Slime who got lucky being a demon lord and told him not to operate in their affairs and they will not either. After the tournament, however, her view of him has changed and caused her and her clan members to kneel in forgiveness for their behavior before. She, however, is infatuated with Benimaru after his display of power and skills and declares herself as his future fiancée which the latter was flustered at the declaration and sudden news.
Benimaru: At first she became fascinated with his skills and powers and after Tempest's first tournament she declares herself as his future fiancée, which flusters him at the suddenness and bold declaration. Later on, when Benimaru was about to ascend to become a demon lord he told Rimuru that he must have an offspring before he can become a demon lord, as after becoming one their reproduction organs are no longer validated for child making. As such Rimuru has officially declared their engagement official to which Momiji is extremely happy for and Benimaru was happy as well, though he was flustered from embarrassment. The fact that Benimaru later completed his evolution proves that at the very least she's pregnant with his child.
Momiji's first encounter with Rimuru did not start off well because she looked down on Rimuru and did not see any benefits of the relationship with Tempest, but after witnessing the tournament and as well becoming fascinated by Benimaru she changed her opinion.