One common theme could be friendship. Since they are both known in the public eye, fanfics might explore their friendship and how they support each other in their careers. Another theme could be collaboration, like working on a project together.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, and there's a good chance you'll find some related to Colleen Ballinger and Erik Stocklin there.
Well, one popular theme might be the two of them working in the entertainment industry in the fanfiction. They could be facing challenges like auditions and dealing with difficult producers, and through it all, their relationship deepens. Another theme could be a sort of alternate reality where they have different personalities than in real life, and how they interact in this new scenario. It gives the writers a chance to explore different aspects of their relationship and the characters they could potentially be.
I'm not sure of specific fanfics for them, but you can start by looking on popular fanfic platforms like Archive of Our Own. There might be various stories exploring different relationships and scenarios involving Colleen Ballinger and Erik Stocklin.
In the 'the lyle and erik menendez story', Lyle and Erik Menendez are brothers. They are known for their heinous act of killing their parents. Their case got a lot of media attention and raised many questions about family and crime.
Since it's a fanfiction, Erik Lehsner could be an original character created by the author. He might be a fictional being with certain special traits or powers that are important to the story. Maybe he's a supernatural entity, like a vampire or a wizard, which would add an interesting dynamic to his relationship with a human.
Erik is likely not based on a true story. Many such characters are invented to entertain or convey certain themes and messages, rather than being rooted in actual happenings.