If they're complex, I have a hunch they might be weirdly similar to us.

By which I mean they will be multi-cellular.

They will have some form of information-encoding molecule akin to DNA.

They will have bodies made up of separate organs, including ones for absorbing atmospheric gases (i.e. lungs), ones for digesting food, ones for filtering toxins, and likely a liquid based transport mechanism. They will also have sensory organs but they might take different forms, although eyes are likely.

I'm on the fence about reproductive organs. They might not reproduce similarly to us at all.

I would expect them to have roughly symmetrical bodies with an even number of limbs.