I'm not sure specifically as it depends on the particular fanfiction. But it could be Chrona, of course. And perhaps there are other characters related to the Chrona universe that the author has chosen to include, like their friends or rivals.
One key theme could be self - discovery. In fanfiction, characters often go through a journey of understanding themselves better. Another might be the power of love to overcome obstacles, as the title implies. Also, loyalty among friends or comrades could be a theme, especially if the chrona part involves a group of characters.
One aspect could be how the concept of 'love is blindness' is manifested in the Soul Eater fanfiction. It might show characters making irrational decisions in the name of love, like Maka perhaps overlooking some of Soul's flaws because of her deep affection for him in the context of this fan - created story.
In this 'love is blindness crona fanfiction', it may be that the author is using the metaphor of blindness to describe Crona's approach to love. Crona, being a unique and often troubled character, might experience love in a way that is different from others. This fanfiction could be an exploration of those differences. It could depict how Crona's past traumas or insecurities lead to a 'blind' view of love, and how interactions with other characters either help or hinder Crona in seeing love more clearly.
One idea could be exploring what happens when a character who was once blinded by love finally sees the truth about their partner's flaws but still decides to stay. Another could be a story where two characters fall in love in a situation similar to the concept of 'love is blindness', like in a virtual reality world where they can't truly see each other's physical forms at first.
A possible plot could be a character falling in love with an enemy, blinded by their feelings and not seeing the danger. For instance, a meister falling for a powerful witch who is actually planning to use them for some dark purpose.
You can start by giving them distinct personalities. For example, make one character extremely idealistic in love, always seeing the best in their partner no matter what. Another character could be more cynical but still fall victim to the blindness of love in an unexpected way.
The fanfiction could center around how Crona's dark past affects their relationship with Kid. Crona has had a difficult upbringing and Kid, coming from a more stable background, might have to learn to understand Crona's traumas. The 'blindness' of love here could mean that Kid is so in love that he ignores the red flags that Crona's past might bring up, only to later realize they need to work through them together.
Themes in this fanfiction could include the struggle between vulnerability and strength. Crona is often a vulnerable character, and when it comes to love, which is presented as blindness, this struggle could be highlighted. Also, the theme of redemption might be present. If Crona has made mistakes in the past due to a 'blind' view of love, the story could be about seeking redemption.
For Crona, you can start by exploring their insecurities more deeply. Since they have a complex past, show how that affects their actions and decisions when it comes to their relationship with Kid. For Kid, focus on his strict upbringing and how that clashes with his feelings for Crona. For example, he might struggle between following his family's ideals and his love for Crona.