All pair of Resource Link cards' text always mention another card as the requirement for its ability (e.g. "Root of Evil, Azi Dahaka" text mentions "Bullet Rider, Schedar" and vice versa). In addition, all paired cards' illustration are drawn by the same illustrator and can be connected to form a single image. The Resource Link ability is triggered either by putting one part of the pair in Resource while already having the other half in Resource beforehand (for automatic ability), or by having the paired cards together in Resource at the same time (for continuous and startup ability).
Booster 20: Code:Samsara - Blessing Sky introduced a new variant of Resource Link dubbed Kizuna Link (絆リンク Kizuna Rinku, "Bonds Link"). Unlike regular Resource Link pairs which are comprised of two Z/X of different colors, a pair of Kizuna Link cards is comprised of a Z/X and an Event of a single color and all requires a specific Player Card for their Resource ability.