Life Link

A Life Link (ライフリンク), sometimes stylized as Life-Link, is a bond between two individuals that will cause one to revive so long as the other is alive.

Information
It is unknown how life links are created, but when they are, two individuals have their lives bonded indefinitely. When one dies while the other lives, they will revive, regardless of the circumstances regarding their death. Not even Observers have the ability to recognize the death of someone with a life link - the only way for a duo with this bond to perish is if they both die at the same time. Nu-13 was able to resurrect herself even though she had been thrown into the Boundary. During the smelting of Noel Vermillion into Mu-12, Hazama took advantage of the situation to form a life link with her.

It is unknown how Ragna's existence disappearing from the world has effected his life link with Nu. The same extends to Hazama and Noel as the former has entered a different possibility. Those with life links have a state of half-immortality, but it is different from the kind possessed by Rachel Alucard and they cannot be killed by the Immortal Breaker.

Naoto Kurogane and Raquel Alucard share a life link, but it is implied that Rachel and Naoto also share a life link for unknown reasons.

BlazBlue: Central Fiction Library Entry
''The sharing of life between specific lifeforms. If one was to die while the other still lives, it would be impossible to recognise the 'death' through Observation, causing them to continue to live in a state of half-immortality. In other words, it is necessary to kill both parties sharing a Life Link at the same time, or they will simply revive.''
 * Life Link

Presently, those sharing a Life Link are Ragna the Bloodedge with ν-No.13-, and Hazama with Noel Vermillion.