From UFStarfleet Wiki
The Annari are an aggressive and somewhat deceptive species from the Delta quadrant. The primary ruling species of the Annari Empire, they control multiple worlds and subject species. While they prefer to gain the trust of a species, then betray them to annex a new world into their empire, they are able and willing to resort to military force.
Physically they are comparable to humans in regards to size and physical strength, with skin, hair and eye colors ranging most of those found on humans as well. Though greenish brown skin, with brown or green eyes, and brown or black hair is most common.
First contact was made on stardate 54274.7 by Captain Kathryn Janeway of the USS Voyager while undergoing repairs on the surface of an uninhabited world in the Annari Empire's territory. Originally helpful to Voyager, the Annari became irritated, and demanded the Voyager leave Annari space immediately when it was discovered members of the Voyager crew had aided the Kraylor, enemies of the Annari Empire.
Very little is known about Annari culture, however their use of deception and betrayal to gain control of new worlds and territory, and their willingness to blockade non-compliant planets would suggest some level of cultural aggressiveness.
Technology is generally equivalent with the Federation, however they have developed specialized sensors that are able to detect and highlight cloaked vessels.
The Annari will remain helpful so long as they do not perceive you or your ship as a threat. This can include simply having the wrong species aboard as crewmembers, or having aided one of their enemies in the past. As they are a multiple-system spanning empire, they are to be considered dangerous if angered and caution is recommended with any interactions.