Apparently the reason why Yunifer and her dad's relationship is so tense is because around the time that she was 7/8 yo, he thought there was a chance that Yunifer wasn't his child, especially since she was a water mage and he was an Earth mage and her mother wasn't a mage at all. He loved Yunifer a lot but he basically pushed her away really roughly. Around the time that she was 11 or 12 (or some age later), there was a professor who published a paper at the academy that said that the type of magic a person had wasn't inherited. The chances of a child being a water mage or wind mage from an earth mage was possible since there wasn't a major clash between the elements. When he found that out, he tried to reconcile with OG Yunifer and she ended up just ignoring him. Then she went to the academy and the MC became Yunifer so yeah...there's that...