JK Rowling said she would have made Seamus/Dean canon but she felt it would be distracting from the main trio
Literally how much space do you need to have a line about Dean asking Seamus to the Yule ball
Look, I’ll try
"Parvati had tried to ask Dean to the ball, but he told her he was going with Seamus instead."
Seamus and Dean is canon, you can’t tell me otherwise.