One common theme is self - discovery. The OC often discovers new things about themselves while being with Killua. Another theme is friendship evolving into something more. For example, they start as friends or allies and then gradually develop romantic feelings. Also, there's the theme of overcoming differences. Since the OC is not from the original story, they usually have different values or abilities than Killua, and they have to overcome these differences to be together.
To be a good listener, you need to show genuine interest, nod and make appropriate responses to show you're engaged. Also, don't be too quick to offer your own opinions until they've finished speaking.
One popular theme is their friendship. Their bond is very strong in the original work, and fanfics often expand on it, like how they rely on each other in dangerous situations. Another theme is adventure. Since they are both on a journey in the original story, fanfics may create new adventures for them, such as exploring uncharted lands.
A professional counselor or therapist could be a great option. They are trained to listen empathetically and offer helpful perspectives. They provide a neutral and safe space for you to share.