Sven's people are a race of aquatic humanoids that evolved on the same world as the Mantrisses.
Externally these people and Humans are very similar, but the similarity is also internal. This species bodies work along similar lines to that of humans, their brains are very similar, and the two species have practically identical reproductive organs.
There is some obvious similarities, they have slit-pupil eyes, no visible hair, and broad flat noses. As well they are able to breath water, and fins on their back and legs.
Sven was of the opinion that human women are more beautiful than the women of his species, but as no full-blooded females of his species have been seen there is no way to determine if his statement is accurate, or just opinion.
The civilization of these people is much younger than that of humanities, they have not wasted their energy on destructive wars, and have had help from their neighbor species the Mantrisses. Thus they are far more technologically advanced than humanity.
The species level of technology is much greater than that of humanities, they have mastered smaller interstellar craft and have floating probes.