There aren't, as far as I know, any alignment restrictions on characters and their familiars. However, it would be somewhat questionable RP to have a Paladin with an inherently evil creature as its familiar.
"Demons" in the D&D sense don't really exist in Layonara, though the term is probably used by the common folk as a general term for beings that come from outside of the Mortal Coil, and more specifically from the Pits of Endless Strife, but at the same time, not all creatures from the Pits would be considered "demonic". Please see our
Cosmology information, which is our version of the D&D Planes.
Keep in mind also that most of this information is not widely known to anyone in an IC sense. If anything is known, or at least believed, there is an inherent belief in the Heavens (though not the details of their nature) as the house of the gods. As well, the Pits have made it into the common vernacular, mostly as a curse or an oath (i.e. What in Strife was
that), and many users of magic know that those things they summon come from the Pits or one of the four elemental "planes".
There is precious little information available to most, even within magic circles, as the act of traveling to the Pits takes special preparation and skill that only a relative few possess. It's also extremely dangerous.