What fusion reactions other than Hydrogen are important in stars?
Other than Hydrogen and Helium, Carbon, Nitrogen and Oxygen are also important for fusion reactions in stars.
Main sequence stars are fusing Hydrogen into Helium.
Smaller stars, like our Sun, mainly use the proton-proton chain reaction which fuses Hydrogen into Helium via Hydrogen and Helium isotopes.
There is another reaction which is only a minor process in smaller stars like the Sun but very much the main process in larger stars is the Carbon-Nitrogen-Oxygen (CNO) cycle.
The CNO cycle uses Carbon as a catalyst for the fusion process for fusing Hydrogen into Helium. It is very efficient at the higher temperature found in the cores of larger stars.