Erica and Godou started off on the wrong foot, with Erica mistaking him for a mage from a rival magical society. However, his courage and willingness to sacrifice himself, as well as his ladykiller ways, caused her to fall in love with him. Erica at some point stated that she loves Godou for his honest, clumsy, boorish, but yet brilliant and naive nature. Godou was the first man Erica had loved and the one she wants to be by her side forever. However, this affection seems to be one-sided most of the time as Godou constantly fights against showing anything but normal friendship to her. Erica, however, knows from within Godou heart that he does love her from the kiss they shared, which later turns out to be correct. Erica has declared that she will be his wife, though she doesn't mind if he has other women as long as she is #1 in his heart. Erica even has shown signs to willingly to leave her mage association for Godou and wanted him to create his own. While Erica does trick Godou into doing things he does not like, Erica does believe that she is doing it for what's best for Godou and wanted him to be a stronger man to face the world. Erica revealed that while she did cheat in a game with Godou several times, she does, however, perform that because she wanted Godou to feel normal in some aspect and knows it does make him happy. While she doesn't like Godou using Youth Form and becoming bolder with his feeling, Erica, however, stated that she finds it harder to resist him and give in to his command. Erica, despite getting furious at Godou ladykillers way, does believe Godou will not leave her and knows he will continue to love her, whether he wanted to admit it or not. By the end of Series, Godou and Erica became close enough to have sex and even ask Godou to think up a name for their future child.
The pair first started out as antagonists or opponents, as Yuri attempted to control what she considered Erica's unseemly behavior in showing her affection for Godou all the time. After their various adventures together, the two have become more along the lines of friendly rivals for Godou's affections. Though the two still argue over Erica's public behavior. Out of all the girls in the group, Erica was the most willing to let her be Godou mistress since her ability had proven useful for them and believed she was easier to control than the other girls. Erica tends to tease Yuri and have her kiss Godou with a wink.
Liliana and Erica are old friends and rivals from two different orders of knighthood, the Bronze Black Cross and the Copper Black Cross, and they are the only members who can stand toe to toe with each other in skill. However, they are otherwise complete opposites. Liliana is a witch who is very conscious of duty and formalities, while Erica is a Hermetic mage who is much more relaxed about them, though both have a very strong sense of honor. The relationship between these two have gotten closer since they became Godou's woman. They would often work together to support their king and lover, Kusanagi Godou. By the end of series they work together in Shanghai to make a mage association under Godou command and join forces with another association under queen of material art. They even became comfortable enough with one another that both of them were willingly to have sex with Godou at the same time when he returned.
The two have something of a rivalry because of Ena's near defeat of her while in the Netherworld, and because of her romantic interest in Godou. While Erica does forgive Ena for her harsh treatment, but she was not at first accepting of Ena being one of Godou lovers and considers her as the toughest rival in the group because of her innocent and hard to control nature. By Volume 14, however, the two girls are more comfortable around each other and were willing to let the other one get Godou attention if he desires to and would not complain. Erica even shown signs of teaching Ena ways to seduce Godou and treated her well and respected her desire during her and Ena Youth Ritual with Godou.
She is Erica's maid and friend.
One of Erica's fellow members of Copper-Black Cross, Genaro was the only other member to be in line for the title of Diavolo Rosso, the Great Knight of the order. The two appear to hate each others guts for unknown reasons.
Erica has a certain amount of respect for Lucretia, as they both have similar personalities, and Lucretia seems to try and promote Erica's romance of Godou since the three first met but from time to time, desire to be with Godou as well (whether joking or not is unclear) which Erica tend to not take well at times.
Erica on rare occasions when she seeks information that her own sources cannot get, she seek Amakasu out and try to get information out of him.
Erica’s uncle and her guardian, the only person that she listens to. Deep down, Erica sees Paolo as her father figure since her parents died when she was younger and came to respect and idolize him.