Okay, so my character is relatively short (5'6 feet). Her bones are hollow,but similiar to a birds-very dense to make up for strength. Her wingspan is 16 feet. I want her to stay as humanoid as possible. What I'm wondering is if a gas such as helium or hydrogen (or any gas lighter than air) were inside of the hollow bones, would that allow for less muscle density and easier flight? What would the advantages and disadvantages of it be?
We need to estimate how much hydrogen she has in her body. An adult has about 2l of bone marrow, I'll allow for your humanoid to have more space in her bones for hydrogen (since biochemical processes can't produce helium) say she has about 4 litres of H2 in her body.
Now hydrogen has a lifting power of 1.2 kg per cubic metre, or 1.2 gram per litre. So these four litres of hydrogen decrease her effective weight by about 5 grams... not much.
For hydrogen gas pockets to have much effect, they need to be big! Her wings might provide space for H2 to be kept. A 16ft wingspan allows for some extra space, however it is difficult to imagine enough H2 to significantly lighten her, without distorting her wings a lot, to say nothing of the obvious fire hazard of holding several cubic meters of highly flammable gas inside her body. There's a reason why no animal has adopted lighter-than-air flight.
The key to flight is air density. More gravity makes the atmosphere denser, and flight easier.