Before any other, there were the Eldar. They are among the sectors oldest residents with a political landscape so baroque, it defies comprehension. Imperial Xenos scholars come to the Agrippa Sector in droves, to study the ancient Eldar pre-history and maintain that the various groups that inhabit the sector are among the oldest in the galaxy.
Even the language of the Eldar is old. The Ondilinde and Sethnor dialects are so different, that Craftworlds on the other side of the galaxy would be hard pressed to even find them intelligible and indeed relative newcomers such as the Haras ag Iadonna have had strained relations with their kin, perhaps due to difficulties in translation. This has led to the overwhelming assumption that the origins of the Eldar in Agrippa occur shortly before the birth of the Eye of Terror and that the sector was a major pocket of refugee Eldar groups fleeing the great fall.
Before Man arrived there were three thriving Craftworld populations of the Ondilindé and Athenae as well as one planetary based group known as the Sethnor. There were also hundreds of smaller exodite groups inhabiting worlds throughout Aachen and Scylla. Much to the chagrin of the Imperium many of these groups still cling to these territories.