Ivadairy

It's because her past is being slowly told, so we only see her personality weak but not why. I understand she's not you cup of tea, but the definition of strong FL can vary, I personally see her as s strong character because of several criteria.
One, does she have a goal? Yes, mc wants to make the drug to help ml. 2, does she stand by her choice? Yes, she refused the marriage until no alternative was possible. I don't like stubbornness that is thoughtless. 3, is she swayed by her emotions? No, mc refused the marriage although she loves ML. Being vocal about emotions, having the., or crying alot isn't a weakness. It is allowing them to be the only moral compass or personality of MC.
So she is a strong FL in my books

1. She doesn't set boundaries making it easy for the ML to manipulate her, 2. She
let's the ML step all over her, 3. When things don't go her way she runs to the ML with her tail tucked between her legs, 4. She has little self respect for herself that if the ML told her to jump off a cliff she would, in fact, jump off that cliff and 5. The crying, she crys too much, moderate crying is understandable, especially when the circumstances call for it, however the FL crys excessively making her seem like crybaby that can't handle her own. It takes away from any ounce of maturity she could have had and makes her seem naive and childish, Then you have her constant acts of using the overused excuse of how much she loves the ML, So that he can treat her as a doormat however he pleases, It simply makes her look like a weak, codependent people pleaser with low self-esteem that has serious boundary issues. Personally, Like the OP said, I would also consider her to be a mentally weak FL, Your standards for what is considered a strong FL seem very low, no offense lol.

THANK UU I just couldn’t pin point the exact reasons why i just couldn’t get into the story but u worded my thoughts out perfectly .No offense to anyone whatsoever my bestie loved this story but for me the main leads like fl and ml are the most important characters and that’s why i also hold high standards for them .And sladly i just can’t be satisfied with just a hot male lead and a pretty art style for me a manhwa needs to do way more than that

Yeah ik, it is also not my cup of tea, I dropped reading the novel way before the manhwa was released for such reasons, but do I still think mc is not a strong FL? No. Just shitty writing and inconsistencies in the writing.

But she is very devoted because her mother brainwashed her, so I get its putting you off, and has no self esteem because of that mother. ╮(￣▽￣)╭
So... it's debatable wether it's simply weakness embedded to to be healed or is this going to stick, if the story had better writing we coulve been looking forward to the "healing" part, but no, we only get psycho ml and doormat template.