10 Most Shocking Betrayals In Star Trek
8. Ishara Yar - Legacy
This betrayal truthfully falls under upsetting far more than it falls under shocking. Ishara was honouring her peoples' struggle but the cost that follows is something that stabs directly in the hearts of all fans of the Next Generation.
The Next Generation's fourth season is arguably its strongest, featuring many of its best stories. Legacy was the next step in Data's story with what turned out to be the Yar family. Tasha Yar had long since died, though her backstory had been briefly explored. She had been raised in a failed Federation colony, overrun by gangs and violence - escaping all the way to Starfleet.
Her sister was not so lucky.
There is a trope in fiction to introduce secret family members once a story becomes stale. Ishara Yar is an exception to this rule. While yes, she is a secret sister that the audience knew nothing about - it doesn't feel forced. She seems to develop a genuine relationship with Data.
The audience would have been well aware of his relationship with Tasha. While he was free from emotion, there was no denying the closeness that he felt with her. Meeting Ishara then would, if not evoke a sense of grief in memory, at least awaken the same curiosity that he may have felt with Tasha. For Ishara then to use him as a pawn in her plot was heartbreaking.
Data is perhaps the most innocent of any character in the franchise and would have had no comprehension of affairs of the heart. Ishara uses this to her own ends. It's successful.
It's despicable.