Kestrel is part of the Fast-Response fleet of PACT, the Planetary Alliance for Co-operation and Trade. The Alliance is between the seven races who have warp drive (fast-than-light travel). One of the races is human. I have discussed the Altairians, the Anak and the Kohathi so far. Let me introduce two of the others.
Planets: Ochra and Caspar
Ochrans and Casparans look like humans with green skins, but otherwise look alike. Except Ochrans are on average 2 metres tall and Casparans are about one and a half metres tall. There is no love lost between the two races, even though their worlds are both members of PACT.
The Ochrans conquered Caspar and oppressed the Casparans, treating them as servants, and consider them less intelligent. Only 70 years ago the Casparans fought successfully for their independence, negotiated by PACT. The resentment and the contempt is still there.
Ochran and Casparian skin goes pink in an oxygen-rich atmosphere. In Intruders Desmar Barok wears a mask around his neck which he breathes from periodically to keep his skin pure. Merari Two, the Ochran, doesn't bother, and his skin gradually turns pink.
Ochrans are courteous and helpful, but quite formal in the way they behave. Merari's designation as Two indicates he is the second son in the family. He is the Ochran representative in Intruders.
Barok, the Casparan representative, is the son of the Prime Minister and seems to have nothing to offer but his own opinion - on everything. His sole brief is to ensure the Caspar don't get left out of anything or short changed. Because of their heritage of oppression, they are fanatic about rights, and never being under the control of anyone else again.
Since independence, the Casparans are not free of the Ochrans, because they control Casparan trade routes. There is a nebula which blocks other routes, so they have to fly through Ochran space.
The Caspar store their dead in underground vaults, they do not agree with cremation, they think it dishonours the dead. This is a vital clue in a later book in the series.