My neighbor Kazu-chan recently booked a trip to Bali with her friend on Garuda Indonesia. She said the travel agency she booked through charged her ¥3,000 to sit next to her friend. And her friend also had to pay ¥3,000 to sit next to Kazu-chan. This was called a pair seat.
Like a piece of day-old mochi, I found this a bit hard to swallow, so I called the travel agency myself. They confirmed that yes, this charge was for real. And it was an agency surcharge, not an airline surcharge. I wonder how Garuda Indonesia, not a budget airline, feels about this.
The ¥3,000 surcharge is applied even if you only want to sit with the person one way. This makes a "Narita divorce" (where couples divorce after the results of their honeymoon) even more expensive. I wonder how much you have to pay to cancel the pair seat.