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 1.2kg per cubic metre, or 1.2 per litre. So these four litres of hydrogen decrease her effective weight by about 5 grammes.... 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.

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.