Kizuna (絆 kizuna) is a mechanic introduced in WXDi-CP02 Blue Archive DIVA. It is associated with that set's Blue Archive collaboration cards: SIGNI with the Blue Archive class, as well as that set's LRIG types, Azusa, Neru, Shiroko, Hina, Make-Up Work Club, C&C, Foreclosure Task Force, and Prefect Team.
The effects of certain cards allow you to bond/start a relationship with a student of your choice. To do this, declare the name of a student from Blue Archive (a character, not a card name). When you do, all of the kizuna abilities of cards (represented by a heart symbol: 
) depicting that student become active. Declaring a student's name use to cause the kizuna abilities of multiple different cards to become active; for example, declaring that you are bonding with "Ajitani Hifumi" causes the Kizuna Enter abilities of both Ajitani Hifumi "Help Me, Peroro!" and Hifumi Ajitani "Swimsuit" to become active.
However, the current rebalance upon the release of the Blue Archive Starter Deck and Blue Archive SELECTOR now only bonds between different outfits variants, meaning both Swimsuit Hifumi and Ajitani Hifumi are now mechanically treated as separate characters with their own separate relationship tokens, similar to the game they come from.
Once a bond is declared, the bond lasts for the rest of the game. You may have bonds with multiple students active at once.
