There are two things Simon Tam would fight to the death for: his sister and his home, in that order. Series prequel, including all the crew and the special hells they put themselves through. Much Zoe/Wash, other pairings are pre-slash (Mal/Simon, Inara/Kaylee).
Mal, Simon, and River are stranded on a planet after alliance presence makes it dangerous for Serenity.
Simon knows about the Goodnight Kiss first-hand. Pre-series.