Summary:

Bodiless consciousnesses that roam the universe, messing with its very fabric to some extent to fit their desires. They are all connected to the universe differently, so they all have different “powers”.

Biology:

No one really had a chance to dissect one so far (probably because they don’t have actual bodies), and they don’t seem to be open to explain how exactly they “work”, so they’re still a biological mystery.

They are invisible to the naked eye most of the time, but they can give themselves a temporary body/form to make themselves visible, or to at least not destroy the mind of those who can see them when they aren’t actively showing themselves.

Additional facts:

A fantastical book back in my home planet, called the holy bible, mentions angels, describing them as a mess of eyes, rings, bright light, wings, etc. Some historians in Critonk think that might be because, when seen to the naked eye without taking any form, the mind of those rare few who can see them without their consent, tries to make sense of their presence by filling in the blanks with forms that are familiar to the viewer. The book separates god and angels, defining angels as messengers of God instead of gods themselves; this might have happened due to a miscommunication between the planet’s angel, and their “prophets".

The majority of angels present themself with horrific or fear-inducing forms even when trying to communicate with humans, but others take the form of normal humans to do so.

They control everything on the planet by choosing who rules its main kingdoms and reserve the right to dethrone a royal family if they prove themselves unfit. They also try to control the rulers of smaller kingdoms and civilizations to a small extent. Still, they don't seem to have a hand in how Fauna's creations rule themselves, mainly because the majority of them have democracies and good anti-corruption systems and because they don't care much about what the angels have to say. 

Personal experience:

None. I think. Some of them make themselves look like normal humans, so I wouldn’t be able to tell if they walked past me…