A bit of explanation about the Wyrmlings, aka, the Baby Giratina of my Obsidian Mew AU.
So! To start with, the babies are not physical beings. Rather, they're energy beings, formed from Ruler's power, and gain experience and grow inside the Distortion Realm. They start real itty bitty, about a foot long, and grow slowly. They look and feel like smoke, and their wings and spikes are simply mimics of Ruler and their older siblings' shells.
What do they eat, you may ask?
Well, they eat energy!!! :D
That's right, these little guys love to eat energy! Gaia energy is tasty, but they love to eat corrupted energy! They're able to consume and convert it safely into their own bodies, which let's them grow their physical shells as they age. They're like cleaner fish, nibbling off the bad energy from older siblings that come back to the realm, or nibble from portals and pools as they can't leave the Distortion Realm.
They would dissipate if they did, their bodies unable to differentiate between their "selves" and the "world". Without an achor they'd return to being plain old energy. Older Giratina gain a physical shell which protects them from that, but it takes about 50 or so years for that to form naturally.
There are exceptions, but it's mainly a difference of experience gaining.