In S04E15 of Supergirl, Nia Nal kisses Brainy, but after that, Brainy tells her that they can't be together. Why can't they be together?
2Because happy relationships make for bad drama? ;) Count the number of main character romantic relationships that lasted on Supergirl. It shouldn't take long.– Obie 2.0Mar 31, 2019 at 21:24
Because Brainy is from the 30th century, and knows his history. He presumably knows exactly who Nia is "supposed" to be with in the timeline in order to eventually continue the line of Dreamer all the way along to the Legion of Superheroes in his time.
If he is with Nia, then he potentially derails that bloodline and erases his friend from existence. Or possibly he is his friend's many times removed ancestor and he just doesn't know that - yet.
Brainac V is from 900 years in Nia's future. If she had descendants traceable for 900 years until Brainiac V's era, she is likely to have a vast number of descendants in Brainiac V's time, not just one person. Nia is likely to have at least 30 gnerations of descendants in 900 years and thus at least a billion descendants in Brainiac V's era. And few thousand years later Nia and her historical spouse will be the ancestors of every single living human. Risking preventing Nia from finding her historical spouse is risking genocide. Apr 2, 2019 at 18:32
The finally got together in the season finale... Don't know what changed, but fans are happy!– SujitJun 3, 2019 at 3:31