In the episode "Departure", it was discovered that Tess ended up killing human Alex Whitman in her pursuit of decoding the “destiny book”, overusing her mindwarping powers which turned Alex's mind to mush, which killed him. She had got pregnant by Max and fled Earth once found out her treachery.
In the episode "Four Aliens And A Baby" it is discovered that Tess gave birth at her home planet,returned to earth, and by the near end of the episode, she blew herself up to sacrifice herself for her son and the other alien hybrids' (Max, Isabel & Michael) safety.
Tess has a New York duplicate named "Ava"
You: "Oh yes!"
Me:"I wasn't expecting Tess Harding to be the alien who killed Alex!"
You: "In agreement!"