First, give your OC a clear backstory. For example, if they are from a noble family in the Pandora Hearts world, explain what their family is like, their upbringing, and how it has shaped them. Second, define their goals. It could be something as simple as wanting to protect a certain character or as complex as trying to change the whole system of the world. Third, give them unique quirks or habits. Maybe they always twirl their hair when they're nervous or have a particular way of speaking. This will make them more memorable.
To develop characters in your Pandora Hearts oc fanfic, start with their appearance. Describe their clothing style, hair color, and any unique physical features. This gives readers an initial impression. Then, think about their relationships. Who are their friends? Enemies? Lovers? These relationships can drive their actions and decisions. Also, consider their fears and weaknesses. A character who is afraid of the dark but has to venture into the Abyss for a reason is more interesting than one without any fears. Their journey to overcome these weaknesses can be a great part of the story.
Developing characters in a Pandora Hearts oc fanfic requires a deep understanding of the world they exist in. Make your OC react realistically to the events and situations in the Pandora Hearts universe. For instance, if they witness a battle between Chains, how do they feel? Are they scared? Intrigued? Their reactions should be in line with their personality. Additionally, let their character grow throughout the story. Maybe they start off as a naive character but through their experiences, they become wiser and more capable. You can also add internal conflicts, like the struggle between their loyalty to a friend and their own self - interest.