When in doubt, flee.
Standing a little shorter than the average human, breathing methane instead of oxygen, the Unggoy used to be part of the Covenant Empire before it shattered. They were late additions to this empire, but were widely spread for their use in combat. However, this was not always the case as Unggoy, named 'Grunts' by humanity, are an ancient race, which existed well before the Halo event which led to their temporary extinction. By the time the Halo Arrays were fired, approximately 100,000 years ago, the Unggoy civilisation had established into a Tier 6 society, about equal to 19th century humans.
After the Halo event and the subsequent reseeding the Unggoy civilisation slowly rose to their former level. When the Covenant Empire, consisting of 7 species by this point (San'Shyuum, Sangheili, Kig-Yar, Lekgolo, Yanme'e, Jiralhanae and Huragok), took in the Unggoy to become their 8th member in 2142, they had yet to return to their former stage. Because of their relative low intelligence, the Unggoy essentially became a slave race, while still largely retaining their cultural identities. Their membership to the Covenant have not been completely quiet, as the Unggoy have rebelled against the Covenant at least sixteen times between 2142 and the Great Schism. Over time there has grown a large disdain between Kig-Yar and Unggoy especially, and has been the reason for many fights and rebellions.
One of these revolts truly changed the Unggoy's place in the Covenant, when the Covenant High Council refused to investigate the allegations the junior staffer Ord Casto about a plan by the Kig-Yar to poison Unggoy. While this poisoning would not result in the Unggoy's death, it would make them sterile and thus end the Unggoy race. The resulting Unggoy Rebellion was highly vicious and eventually had to be put down by the appointment of an Arbiter and the threat of glassing the Unggoy's homeworld Balaho. The Unggoy proved that they were not the cowards they had been long deemed, and could indeed be vicious fighters when provoked. After the uprising's conclusion, as was their tradition, the Sangheili forgave the surviving Unggoy and allowed them to enter the Covenant military as armed infantry units, rather than the unarmed cannon fodder as which they had previously served.
The life of an Unggoy
Now that the military life was open to the Unggoy, many of them entered, receiving the rank of Unggoy Minor. Mostly wielding either the Plasma pistol or Needler, with Plasma grenades, they are proficient fighters. While easily dealt with on their own, a (large) number of Unggoy are capable of dealing with many threats, including human Spartans. Although usually commanded by a Sangheili, Unggoy can also achieve higher ranks. Their highest possible military rank is Ultra, which gives them command over groups of lower class Unggoy, as highly ranked as SpecOps, but does not give them power over other Covenant species. Sangheili or Jiralhanae are ultimately in complete command; in the same way that even minor San'Shyuum outrank Sangheili Shipmasters.
While Unggoy have fought in every (major) war the Covenant has fought since their inception, the Unggoy are not just fighters. While the Covenant Governmental system is mostly Sangheili and San'Shyuum, Ministry of Tranquility offers a single rank open to Unggoy, Deacon. Deacons serves primarily for morale and religious purposes and are sent to help reinforce the spirituality of ships in which little is present. They are taught in the religion of the Covenant, work to increase faith in others and are learned in the "Sacred Glyphs" that the Forerunners use in their machines. While highly respected amongst Unggoy, Deacons are still highly mistreated by other Covenant races, especially the Kig-Yar. When Deacons are deployed to military ships, they are given appropriate armour, highly similar to the armour of Ultras. Outside of combat situations, Deacons tend to be garbed in a ceremonial tunic.
The other profession which is available to Unggoy is merchantry, so far the only Unggoy occupation which is well-respected in the hierarchy. Together with a Yanme'e Queen and Kig-Yar ship masters, Unggoy Merchants were present at the induction ceremony of the Prophet of Truth. It was stated that these Unggoy were so successful, that their Combat Harness was adorned with large jewels. Merchants have been known to trade human audio transmission, like soap operas and radio chatter, on the Covenant black market.