Unless you make a material named light, no.
The only way to make a creature emanate actual light is to use LIGHT_GEN, which is a creature tag for creatures.
Though you could use fire instead.
EDIT: and no you cannot make something lighter than air, unless your using a gas of some sort.
OK will have to make do with fire thanks.
Using this information I will make a Fun O
verP
owered Summoning Creature current nickname "Shadow"
It takes the form of a tallish but skinny human made of a very light fragile flesh covered by a pitch black taut skin, they bleed light
for now it will be fire that does not burn they cannot wear armor
to balance this they move very very fast do not tire do not have bones to break are very very strong and start with great fighting stats They use Twin swords that are "better but different" than adamantine
Calling them Torivorian Steel Swords, Torivorian Steel is very very heavy (a single sword is as heavy as a great axe) but has twice the sharpness as adamantine ( they are just a bit more valuable too). Torivorian Steel Swords can only be gotten by killing a shadow They are mostly Nonagressive however they will attack when you try to hurt them usually with fatal consequences. Shadows do not heal normally however every month at new moon they will full heal. Summoning a Shadow Nearly Kills the Caster but if they the use a dead corpse (any) it is only taxing.
Anything else I could add to make it cooler? Or Else Flesh out the idea?
So could I make a gas that's lighter than air then use it to fill something?
Thus reducing it weight to less than air.