My two cents? Erynn's story is just another story in the VN. She does have a distinction of being involved in the reason that MC started misbehaving, which is the real reason he ended up living with his father. But Alexis was there too. So Alexis' story is inseparable from Erynn's story. This is the way for most of the stories, but especially these two.
So why is it that Alexis' story is so much better than Erynn's? Why is it that Alexis is capable of a relationship with its ups and downs and Erynn is just kind of a drag? I don't think that it is necessarily Erynn's character. She is a perfectly lovely person. It is because people don't really know her deal. We don't need to know everything about a character, we just need to know their deal. Alexis wants MC to be her lover, and does the things to make that happen. What the hell does Erynn want?
Everybody knows by now that I'm not a fan of Erynn's story. It is too much of an ask that the author is imploring us to buy that she was carrying a torch for MC all the time he was gone, especially since he was giving her the gray rock treatment. Human beings don't function like that. Any normal person who was ignored and given the cold shoulder for years would naturally have their feelings toward that person change. Not for the better.
So the author is asking us to buy that Erynn loved this little kid. And now she is well into adulthood and he is just on the precipice of it. Despite being at dramatically different places in life and not really knowing each other after being apart for so long, she loves him with that love that is too embarrassed to speak its name. For most people, this just doesn't pass the smell test.
Alexis' story is a little more familiar if also formulaic. We are presented with what they call a meet cute. Even though they know each other, they don't really remember. Freed from this particular burden by the kindness of forgetfulness, they are able to treat their relationship as something new. Alexis is also attractive because she is optional. You can be rude to her and she disappears as a normal person would. Or you could be kind and love blooms. The past eventually intrudes, but that only happens once the reader is invested in the relationship.
Erynn doesn't seem optional, but she also isn't given good material. She always just gets shoehorned in. MC is walking around with a tack in his shoe and that tack is named Erynn. No matter how he once felt about her or the emotions that re-occur when he encounters her again, they are different people than they were during that event that was so painful to him and was just another day for her. But they don't start fresh. She wants to pick up where they left off and he wants no part of that. So their story stagnates and never really goes anywhere because they are at an impasse. They want different things, and the reader cannot buy into her because from this point on everything in her life is a contrived mess. She is a bottomless pit of need and MC has the feelings of so many other women in his life to consider.
What do you want Erynn? Do you want to be a model or a kindergarten teacher? Do you want your childhood friend back or are you in love with the man he became? Because MC needs to know and he doesn't have that much time to figure it out. He has to choose between two different sets of sisters and a gaggle of classmates. Things are weird with his mom and your mom too. If you want to work here, close the deal! Tick tock, Erynn. Let's hear your elevator pitch. Should we forget the past or is our history so important it can't be discarded? Hurry up, Erynn. I have a date. My right foot is in the car Erynn. What do you want? Help. Okay. You want help. You need a hero. Swell. I'm a teenager and there are at least two other stalkers of girls that I am dating that want to kick my ass but sure. Let me drop everything and solve your problems. Fuck this shit. My princess is in another castle.
So Erynn's story is frustrating and bewildering to many people. The author goes out of their way to make the MC feel wronged by her and then makes no effort to heal things between them before asking us to let her back into his life. We tend to reject this premise. Even if we do accept her back into his life, all she brings to the relationship is confusion and pain. She is pretty to look at, but she doesn't just have baggage but a baggage train. Almost every guy has a shitty ex-girlfriend just like her and almost every girl has a shitty ex-friend that she reminds her of. Erynn just sucks all the energy out of us because the story doesn't go anywhere. Get past a roadblock and there is just another roadblock. It seems one way but scratch the surface and what do you find? That's right. More surface.
So a lot of people will naturally take Alexis' side if forced to choose. Because it doesn't matter how you feel about somebody. What matters is how you treat them. Alexis treats the MC right. She does lie to him, but she acknowledges it and feels bad about it. You know, like a normal person. She might not be honest about everything, but she is honest about her feelings. Erynn acknowledges no wrongdoing. Everything is someone else's fault or a misunderstanding. She is the victim and everyone is against her. Why can't anyone see the love she has inside? Blah blah blah.
Again, this is a flaw of the story and not necessarily a character flaw. We just haven't got any scenes that have given us any reason to give a fuck. How awesome would it have been if Erynn came up behind Ethan and kicked him in the nuts? How awesome would it be if when MC was getting attacked in the park Erynn was there to shout at the stalker and pick MC up off the ground? What if she was worried about him instead of vice versa? It probably would have felt as forced and arbitrary as the rest of the story but instead, she was off washing her hair or something. She had potential as a character. Instead, she is literally dumpstered because her story is garbage. So if the author wants people to like Erynn, they are certainly going about it the wrong way. She might end up making some guy very happy someday, but her story doesn't sell her as a character we should choose over any other random love interest.