Monkey-king, brother of Vali; with his army of monkeys headed by Hanuman, assisted Rama in defeating Ravana. (Glossary for the Vahinis)
Chronicle of origins and fortunes of Sugriva’s entire Clan:
Once upon a time, Brahma, the Creator, created a monkey form. It was endowed with great might; but, it was ever wayward in movements and activities. So, Brahma named it Ruksharaja. When it demanded to be told where it should reside, Brahma directed, ‘Live in the forest, for there you can move as your waywardness dictates. And when you catch a Rakshasa, kill him and save the area from his misdeeds. Ruksharaja migrated to the southern region, and followed Brahma’s instructions. One day, the monkey Ruksharaja went to a lake to slake its thirst and when it dipped its face on the surface of the clear water, it saw its image in the lake. It was much concerned, for evidently there was an enemy hiding in the lake, lying in wait for him! It roamed all-round the shore of the lake, eager to catch the enemy when it popped out of the waters. The enemy inside the lake roared when he roared, gnashed its teeth when he did so. It echoed, reflected, all noise and all gestures. Unable to control himself any longer, Ruksharaja jumped into the lake to strangle his rival. That jump transformed him into a female! Struck with amazement, she came on shore; and turning to the Sun, she prayed for Grace. She also prayed to Indra, with great mental anguish. Through the Grace of Surya (the Sun) she got a son, that is, Sugriva, myself; and through the Grace that Indra bestowed on her, she got another son, Vali, my brother. Immediately after the birth of the two children, she became once again, Ruksharaja! Ruksharaja took the two babies with him and approached Brahma for instructions. He related to Brahma his entire story so that He could recollect the facts of his decision thus: “Oh Vali and Sugriva! Go into the regions of the South and establish yourselves in Kishkindha. The Lord of all the Worlds, the Supreme Sovereign of the Universe, He who is known by many Names will take birth as Rama, as the son of Emperor Dasharatha of Raghu Dynasty. He will come into the forest according to his father’s command. He will engage himself in many superhuman achievements. He will also behave like an ordinary mortal. During his wanderings, he will arrive at Kishkindha, where you are, and form friendship with you. Seek the fortune of securing his darshan, hearing him speak and touching his feet. Your lives will be rendered blessed thereby.”
“We listened to the Voice of Brahma addressing us thus. We were delighted at the prospect that lay before us. We did not undertake any japa, austerity, ritual or yajna. All our talents and accomplishments were the direct result of the Grace that Brahma showered on us that day. When that Voice ceased, we offered homage in our minds to Brahma and reached Kishkindha. We destroyed the rakshasas who infested the forests there. (RKRV Part II, pp.85-87)
Vali and Sugriva - they too were brothers but both had to undergo suffering on account of lack of unity. (SSB 2000, p.81) In every human relationship, there should be balance and parity. At one time, the brothers Vali and Sugriva were inseparable; but enmity developed and they parted company. While Vali remained in his place, Sugriva hid himself in the mountains to avoid Vali’s wrath. Sugriva was all the time wondering how he could overcome his brother, now his enemy. Vali was very strong and powerful and could be defeated only by one who was more powerful than him. Sugriva consulted his companion Hanuman for advice and the latter suggested that Sugriva should be patient; time would show the way. (SSB 2000, p.122)
Striking personalities of Ramayana:
The portraits of two personalities are very striking in the Ramayana. They are those of Sugriva and Vibhishana. Vibhishana went in quest of Rama, while Rama Himself went in search of Sugriva. Sugriva realized Rama’s greatness and goodness only after meeting Him, while Vibhishana knew of Rama’s excellence even before he met Him. (SSB 1996, p.92)
See also Ramayana Vol.4 - Chapter Sugriva (p.119)