A question came up when playing tonight: are defenders declared simultaneously, or are they declared one at a time? The issue arose when I was playing Walder Frey, and my opponent contended that his passive effect would fire off as I knelt one character (who had lower than the attacking strength, leading Walder to participate on his side), not after I declared the total number of defenders that I intended. Who was right?

Defenders are declared and knelt all at the same time. So Walder does not jump in right after you kneel just one, but after you have knelt all your defending characters, on the winning side

And even if they were declared one and a time, you'd have to complete the entire step for declaring defenders (ie, declare all your defenders) before Walder's passive kicked in.