- "I am a star at rest, my daughter. When I set for the last time, decrepit and old beyond all that you can reckon, I was carried to this island. I am not so old now as I was then. Every morning a bird brings me a fire-berry from the valleys in the Sun, and each fire-berry takes away a little of my age. And when I have become as young as the child that was born yesterday, then I shall take my rising again (for we are at earth's eastern rim) and once more tread the great dance."
- ―Ramandu (Chapter 14) [src]
He was an old man with silver hair and a beard that went all the way down to the floor. Tall and straight, he was clothed in a robe that appeared to be made of the fleece of silver sheep, and his feet were bare.
He was mild and grave of demeanor, and seemed to radiate light, commanding silence and respect.
It is unknown when Ramandu was first born as a celestial star, but after many years he became tired, as all stars eventually do, and returned below to the surface of the world. During his rest, he lived on his own island, located at the beginning of the world's end, where years later he eventually bore a daughter with an unknown human woman.
During his time as an earth-bound star, he and his daughter together guarded the ancient Stone Knife of the White Witch, which she had used to kill Aslan during the Winter Revolution. It lay on Aslan's Table, and was forbidden to be touched by anyone.
In the 2290's NY, however, Ramandu's island was discovered by three of the Seven Lost Lords, Mavramorn, Argoz and Revilian, one of whom touched the knife in a fit of anger, causing all three of them to fall into an enchanted sleep.
In 2306, a ship called the Dawn Treader docked at the Island, searching for the three lords and Aslan's Country. This party was led by King Caspian X of Narnia, who quickly fell in love with Ramandu's daughter.
When Ramandu first appeared to the travelers of the Dawn Treader, he did not greet them, but joined his daughter; with her he raised his arms toward the east and sung a ritual song of welcome to the Sun. Afterward, a Bird flew by and placed a bright fruit into his mouth. This was a Fire-berry, which renewed his strength each day until the time when he would be able to once more join the great dance.
He told Caspian that in order to break the enchantment of the Three Sleepers, it was necessary for someone to sail as far east as possible, and to leave there a volunteer who would continue on to the Utter East. However, he could not give Caspian navigational information about the journey there, because he had only a star's perspective of Narnia.
When Ramandu first told the visitors his name and there was no recognition from them, he realised that he had ceased being a star long before they were born. He also corrected Eustace for equating what a star is made of, stating that although in our world stars are made of balls of flaming gas, that is not what they are.
He also informed the adventurers that they had already met another star, Coriakin, who he indicated was earthbound because of some personal failure. However, he would not reveal the circumstances of Coriakin's fall, on the grounds that humans are not meant to know such things about stars.
Dismissing this line of questioning as useless, he pressed Caspian to decide about sailing east. When the king mentioned the weariness of his crew, Ramandu said that enchantments can be broken only by knowledgeable and willing participants. He also assures them that should the adventurers wish to winter on this island, Aslan's Table would supply them daily with royal fare.
Finally, Ramandu laid his hands, radiant with a faint silver glow, on the head of Lord Rhoop, so that he could experience rest in the company of the three sleepers.
When the crew finally left for the Utter East, they left behind Lord Rhoop and one other Narnian with Ramandu, who cared for them until their return. When the ship eventually returned, they took the Narnians back home with them, including Ramandu's Daughter who left to be with King Caspian whom she had fallen in love with. Presumably, she left with Ramandu's blessing.
It is most likely that Ramandu eventually returned to the skies when he was fully rejuvenated, and shone once again as a full star.