This is everything we know about Sareena so far:
Sareena is a demon from the Netherealm who first appeared in MK:Mythologies. She was one of Quan Chi's assassins but she helped the original Sub-Zero defeat him because she wanted Sub-Zero to help her escape the Netherealm. She was then shot in the back by Shinnok and believed to have been killed.
In MK:Tournament Edition it was revealed that Sareena wasn't killed, but her human form was destroyed. Her essence was banished to the 5th Plane of the Netherealm as punishment for helping Sub-Zero. However, she discovered the portal Scorpion and Quan Chi used in Deadly Alliance and escaped into Outworld where she later found a way to regain her human form without Quan Chi's magic.
After her escape she meets up with the current Sub-Zero and he offered to let her stay with the Lin Kuei in return for her helping his brother against Quan Chi. However, her ending suggests that "trusting a demon from the Netherealm does not come without risk."