The Alien Within
Human controlled space is filled predominately with what we would consider standard humans. Also present are transhuman sub-species, genetically manufactured for specific environments (e.g., “dwarves” on a high gravity world, or “merefolk” on a water world). These (sub)human offshoots would probably suffer some discrimination when not in their “natural” environments, but in most cases they would be considered fully legally human. A notable exception would be a Corporation which specializes in manufacturing transhuman species would definitely consider them property. It would also not be unheard of to find the occasional uplifted species from old earth (e.g., monkeys, dogs, cats, dolphins, etc.) raised to sentience by technology and/or genetics.
The Alien Without
Any non human, non earth decent species would be rare ( but definitely not unheard of) in human ruled space. these aliens would be mostly subservient to humanity. the degree of which would be dependent upon the type of government they were under. An federation-like alliance of worlds (like The Coalition) would treat them with a near equality, while a Corporation with tendencies to strip mine star systems of all their resources (like the Halite Corp) would enslave any aliens they came across.
The Alien Beyond
Outside of humanities sphere of influence there will be definitely be aliens to be discovered and communicated with. Some will be planet bound and primitive, others will be technologically superior and star-spanning. The rest will fall somewhere between the extremes. New and as of yet undiscovered aliens would definitely be grist for the storymill for adventuring explorer player characters.
The Alien Unknown
Just being poetic. Unless there is some truth to that old spacers tale.