The relationship between Patrick and Alex began in 1991. They both just started their education at Hogwarts and first laid eyes on each other when the Sorting Ceremony took place in which Alex was put into Slytherin, and Patrick into Hufflepuff. Although they don't share the same House, a bond was quickly formed between each other when they started to talk more in, and after, class, and eventually they began to consider each other.
Patrick's little brother Liam eventually developed a crush on Alex when he was at the end of his second year and he made Patrick promise not to tell Alex about it because he felt embarrassed. Patrick agreed to his brother's demands but told him that eventually he had to confess his feelings to her in case she felt the same about him. If he did not, and someone else did it before him and she date them instead of him, he would regret it incredibly for the rest of his life.
Liam eventually came clean and confessed to Alex. To his surprise, she'd developed feelings for him as well and the two fell into a relationship. That relationship eventually made Alex part of the Dunbar family and made the bond between her and Patrick even stronger.