Eva is based on the Queen from the fairytale, "Snow White".
Before First Curse
Eva, a princess of a northern kingdom, meets Prince Leopold, whom she has been engaged to since birth, for the first time. However, soon after her arrival, he proposes to a commoner, Cora. Two days before the marriage union, she overhears Cora talking to the gardener, Jonathan. From eavesdropping, she learns Cora is pregnant with his child, and gains insight on how the two met when Jonathan pretended to be a prince to attract her. To buy his silence about her pregnancy, Cora agrees to meet with him later and provide him with jewels. Using this to her advantage, Eva tells Leopold what she has heard. When he confronts Cora, she attests Eva is telling lies out of jealousy to sabotage their wedding. Not to be bested, Eva coolly suggests Leopold should check Cora's pockets for the jewels she was going to give Jonathan. After finding the pouch in Cora's dress, a disappointed Leopold has the guards haul her out of the castle. He then requests Eva's help to break the bad news to the king. As they stop at the altar, she comfortingly assures that he'll have a wife one day who can give him a child as pure as snow. ("Bleeding Through")
Some time following this, she visits the kingdom of King Xavier, and notices Cora, who nearly married her husband, in peasant garb delivering flour. In an attempt to humiliate her nemesis, Eva sticks out her foot and trips Cora, causing her to spill the flour. Cora insults the princess, who is asked by King Xavier if she is alright. Eva states she is fine aside from the fact Cora has ruined her slippers. Cora is made to kneel and apologize, although the accident was not her fault. Later, at a royal ball in which the king's son, Prince Henry, is looking for a bride, Eva and the other ball attendants watch as Cora spins straw into gold in front of the king's court. Immediately after, Henry proposes to Cora, who accepts. ("Bleeding Through", "The Miller's Daughter")
Many years later, Eva marries Leopold. Her husband grows to love her and shows his affection for her by gifting her a brooch, which she treasures. Eva gives birth to a daughter, Snow White, and is hailed as a good and kind queen. When Snow's birthday celebration comes to pass, Eva is proud and excited to give her an heirloom tiara. The two encounter their servant Johanna trying on Snow's tiara. Snow moves to berate her, stating that servants should not wear crowns, but Eva informs her daughter that it does not matter if someone is a royal or a servant as each person deserves love and respect. When she questions Snow about what the birthday celebration is for, the princess selfishly believes it's to honor herself. However, Eva emphasizes that it's for her to honor the people and show them the kind of virtuous Queen she will be in the future. Then, Eva places the tiara on her daughter, beaming with pride, before she collapses from a mysterious illness. After Eva is put on bed rest, Snow wishes to cancel her birthday celebration until she gets better, but Eva insists that the gala must go on without her. The Queen's condition worsens as she begins coughing up blood, to which a desperate Snow seeks out the Blue Fairy for help. Snow returns later to confess that she couldn't accept the Blue Fairy's dark magic because it required her to take someone else's life to save her mother's. Eva praises her daughter for showing strength and sticking to the side of good, and before passing away, she vows to always be with Snow as long as she remains benevolent in her heart. The next day, Eva's body is on display at the mourning ceremony, during which Snow lays a single snow drop on her mother's chest. After the rites are done and everyone has left, the Blue Fairy enters and transforms back into Cora, who then begins talking to Eva's corpse. She admits to killing Eva with poison in order for her own daughter, Regina, to take the Queen's now vacant seat. Cora expresses hopes of blackening Snow White's heart and destroying the legacy Eva left behind. ("The Queen Is Dead", "Heartless")
The casting call describes her as, "Katerina captures the affection of all those that meet her, thanks to her pure spirit and kind heart. As if she doesn't already sound perfect enough, it's worth noting that she treats everyone with dignity and respect, no matter their place in society and she's super close with her adoring fam." 
Actress Rena Sofer wore a wig for her role as Queen Eva due to her natural hair being too short.