Percy, Fred, and George are triplets

The triplets were supposed to come out by March 31 which is what happens to Percy, born on March 31 while his two brothers had a delay and were born just after midnight on April 1st.

Ever since a young age Percy knows he's not like his triplets. Fred and George are both outgoing and rule breaker while Percy is quiet and a rule follower. His brothers at first did try to include Percy in their antics but it never works out because of their difference. Instead they opted to prank on their older triplets instead which is Percy, making him be their target most of the time. At the age of 5 is when they got called Percy and the twins. Percy of course is hurt by this that he's not included because they're his triplets they're not twins but as always he bit back opting to be the perfect obedient son and brother instead.

Percy tried to get along with his other siblings instead, first he tried to get along with his older brothers aka Bill and Charlie but the big age gap made it almost impossible for Percy. So instead Percy tried for his younger siblings which is Ron and Ginny and then once again he just didn't click because of his upright attitude making them not like him. Percy, upset about it tried to make friends instead but it always turns out to be bad because Fred and George are always driving them away or making fun of Percy in front of them or pranking them away making Percy unable to make friends again.

Once they're old enough for Hogwarts the same thing happens again but this time, Fred and George had made something they called 'Percy-posing' which is when they imitated Percy but makes it more exaggerated. This of course made Percy a target to get picked on the most in their year and leave him friendless once again. So instead Percy set goals for himself which is to thrive to be like his older brothers aka a prefect. For years Percy maintained a perfect record plus high achieving academic records as well.

The long awaited day came during the summer break at the burrow the triplets got their Hogwarts for their 5th year, Ron got his for his 3rd year, and Ginny got her's for her 2nd year. Everyone expected Percy to get prefect since compared to the other Gryffindor in their year it's a clear landslide that Percy's the only one perfect for the prefect role. Percy opened his letter to find no prefect badge which ultimately crushed him. The other Weasley surprised by this as well while molly wrote to Dumbledore in case there's a mistake or anything even Fred and George got mad about it. They then got a response from dumbo which just told Percy to meet him in his office on the first day of school as they all know just thought that it was a mistake and Dumbledore wanted to give him the badge himself.