Yu-Gi-Oh! 5D's episode 11: "The Take Back, Part 1"
Blister was in a tag-team Turbo Duel, with Aero in the side-car. Their opponent spun out and knocked Blister off track, causing the cable connecting the two cars to break. Blister tried to save Aero, but pulled away to avoid crashing. Aero crashed into a factory, while Blister spun away, breaking his Duel Runner. Unbeknownst to Blister, Aero saved "Machina Sniper", which was their favorite card, from a blaze, causing him to get hit by an explosion which could have been avoided otherwise.
After the accident, Aero stops riding. Blister never saw him again and felt as though Aero resented him. Aero sent the "Machina Sniper" card to Blister, a gesture Blister believes to be out of spite.
Yusei Fudo finds a picture of Aero and Blister and beneath the "Machina Sniper" that was damaged from the blaze. He later insists that Aero had saved the card, repaired it and sent it to Blister as their "Bond of Friendship".
The only card seen in Aero's Deck is "Machine Sniper", credited as his favorite card.