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. 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 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.
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.
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 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. Erica considers her as the toughest rival in the group because of her innocent nature.
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, and has since the three first met.
Erica on rare occasions when she seeks information that her own sources cannot get, she seek Amakasu out and try get information out of him.
Erica’s uncle and her guardian, the only person that she listens to.