Well, the main reason why people hate Shasa is because she didn't immediately apologize and explain exactly what happened to him when she advised him to take herbs somewhere and that place was the place where the black dragon bathed, which caused Hyce to almost be killed. After that, when Hyce woke up, Shasa didn't wait/accompany him until he woke up or even gave him a letter and apologized, instead she took the mission the next day after Hyce was treated.
2 year skip isn't explained in the manga, but in WN in those 2 years they met several times at the guild but all they did was glance at each other and just leave, making Hyce think they didn't care about him anymore and ignored him. and when they both became S rank adventurers, that's when Shasa wanted to apologize, which MC of course refused because he thought they had hidden intentions towards him. Maybe it's for this reason that people hate this girl so much.
But there are several things that make me unable to think like other readers because their problems are full of misunderstandings. there are some parts that are not explained in the manga according to WN or LN so people quickly jump to conclusions about the girl, ignore the rest of the Secred team because this is just a problem for these two people.
Initially they checked their abilities. Shasa with "Master Sword", Hyce with "Master Weapon" and Hyce doesn't know how to use his abilities, there they are only 12 years old. Before forming a party they trained under Geist for 1 year (their relationship here is still good and Shasa has not been rude to him) and only formed a party at the age of 13 and a few months later they got members namely Raynold, Robin, Tycon, and Piazolla (after there was a new member coming in, Shasa was often rude to Hyce due to various factors and Shasa cried until she fell asleep every day because she was rude to Hyce and Robin said it was because of girl problems) and 1 year later at the age of 14 Hyce was expelled because he was considered incapable accompanied Secred in his current condition (when Rynold asked Shasa if he had made up his mind and Sahsa answered "Yes, because he is a special person" while smiling and blushing (here Raynold didn't like the face that Shasa was showing because every time she discussed Hyce, she was always like it).
There are details (but maybe only a few things because it's going to be long) that I want to cover and you probably won't find in the manga:
- Shasa trembled with fear when she found Hyce carrying a dragon's head and looking at her with strong killing intent towards her, this was the reason Shasa had not been able to apologize for 2 years because she was afraid of Hyce.
- Shasa was confused about how to apologize properly because every word she said meant nothing to Hyce anymore and she did various things to stay connected to him even offering her body to Hyce (Hyce thought she was very pathetic at that time).
- Shasa has always loved Hyce but only realized after Hyce was no longer with her.
- She is always jealous of girls who are close and talk intimately with Hyce (in WN when facing the 4th labyrinth exam part "Ur Hood" namely Robin's older sibling when facing his past he chose to guide and accompany Robin until she was an adult and they met Secred's party where Hyce remains in the party with both eyes looking intact and Shasa is jealous when Robin talks intimately with Hyce) here it is explained that Robin has not entered Secred's party earlier Hyce looks at Secred (and possibly has awakened him abilities). What is the reason for Shasa being rude? (jealous) of Hyce because he often gets along with Robin?
- Shasa started being nice to weak adventurers after having problems with Hyce.
- Shasa is always thinking about Hyce everywhere (what if she keeps Hyce in Secred and when Hyce awakens his abilities the two of them will become adventurers in the spotlight and then become a couple and get married and have children and so on) and her friend knows who she's thinking about.
- and there are many more that if explained would feel like it would be too long.