Who is Rúmil?

[Maedhros]

Who is Rúmil?
Rúmil is a ñoldorian sage of Tirion, 'who first achieved fitting signs for the recording of speech and song' (The Silmarillion). We have as far as I know, no date of his birth, nor a description of his appearance. There is no reference of him being an elf who woke up in Cuiviénen or was born in Aman after the journey there.
What we know is that at first, in The Book of Lost Tales, at the beginning of the legendarium, Rúmil was a philologist and was at one time a thrall of Melkor.
From The Book of Lost Tales I: The Music of the Ainur

Thus it was that the Ainulindalë was first to be heard by mortal ears, as Eriol sat in a sunlit garden in Tol Eressëa. Even after Eriol (or Ælfwine) had fallen away, Rúmil remained, the great Noldorin sage of Tirion 'who first achieved fitting signs for the recording of speech and song' (The Silmarillion p. 63), and The Music of the Ainur continued to be ascribed to him, though invested with the gravity of a remote time he moved far away from the garrulous and whimsical philologist of Kortirion. It is to be noted that in this account Rúmil had been a slave under Melko.

Lets take a look at his alphabet:
Alphabet of Rúmil
From The Book of Lost Tales II: The Tale of Turambar

There is evidence that the Tale of Turambar was in existence at any rate by the middle of 1919. Humphrey Carpenter discovered a passage, written on a scrap of proof for the Oxford English Dictionary, in an early alphabet of my father's devising; and transliterating it he found it to be from this tale, not far from the beginning. He has told me that my father was using this version of the 'Alphabet of Rúmil' about June 1919 (see Biography, p. 100).

From Analysis of a Rúmilian Document by Arden R. Smith

The oldest [Elvish alphabet] is the Alphabet of Rúmil. This is a final cursive elaboration of the oldest letters of the Noldor in Valinor. Only the completion and arrangement of this system was actually due to Rúmil of Túna; its author or authors are now forgotten. Though originating in Túna it is called 'Valinorian' because it was mainly used for writing of Qenya, and was later ousted from use among the Noldor by the alphabet of Fëanor. It is said still to be used by the Lindar of Valinor; but is not in general use among the Qendi.

Based on these passages, then, the following assumptions about the form of the Rúmilian letters (or sarati, as they are called in WJ:396) may be made: (1) they are not "runic" in shape, (2) they must bear at least some similarity to the Fëanorian letters, and (3) they are not written from left to right, which means that they are most likely written either from right to left, in boustrophedon (i.e. with lines alternating left to right, right to left), or vertically, which is in fact how the Rúmilian letters are generally written.

Rúmil is the author of several documents such as: The Music of the Ainur, which later was named Ainulindalë, and the Annals of Valinor, later Annals of Aman. and The Ambarankanta . He is also credited as being an influence in the writing of The Lhammas.
From The Lost Road and Other Writtings: The Lhammas

The ‘Lhammas’
This is the ‘Account of Tongues’ which Pengolod of Gondolin wrote in later days in Tol-eressëa, using the work of Rúmil the sage of Tûn. This account Ælfwine saw when he came into the West

This is the Title Page of Manuscript B. So we see that Rúmil has his fingerprints in other works, that are not his own.

[Maedhros]

Now let’s take a closer look on the Annals:
From The Shaping of Middle-earth: The Earliest Annals of Valinor

In 2993 it is said they came to a place where a high rock stands above the shores, and there stood either Mandos or his messenger and spoke the Doom of Mandos. For the kin-slaying he cursed the house of Fëanor, and to a less degree all who followed them or shared in their emprise, unless they would return to abide the doom and pardon of the Valar. But if they would not, then should evil fortune and disaster befall them, and ever from treachery of kin towards kin; and their oath should turn against them, and a measure of mortality should visit them, that they should be lightly slain with weapons, or torments, or sorrow, and in the end fade and wane before the younger race. And much else he foretold darkly that after befell, warning them that the Valar would fence Valinor against their return.(20)
Felagund and the other sons of Finrod went forward also, for they had aforetime great fellowship, Felagund with the sons of Fingolfin, and Orodreth, Angrod, and Egnor with Celegorm and Curufin sons of Fëanor. Yet the lords of this third house were less haughty and more fair than the others, and had had no part in the kinslaying, and many with Finrod himself returned unto Valinor and the pardon of the Gods. But Aulë their ancient friend smiled on them no more, and the Teleri were estranged.

Note 20: Added here in pencil: Here endeth that which Rúmil wrote.
From Morgoth’s Ring: Annals of Aman

The first has the title The Annals of Valinor, and opens thus: 'Here begin the Annals of Valinor, and speak of the coming of the Valar to Arda'; beside the title was added: 'These were written by Quennar i Onótimo who learned much, and borrowed much also, from Rúmil; but they were enlarged by Pengoloð.' This last was struck out, and the title and preamble emended to the form they have on the second copy, as given below, with Valinor > Aman and the addition of the words 'which Rúmil wrote (made)'.

From Morgoth’s Ring: Annals of Aman: Typescript AAm* preamble

Here begin the 'Annals of Aman'. Rúmil made them in the Elder Days, and they were held in memory by the Exiles. Those parts which we learned and remembered were thus set down in Númenor before the Shadow fell upon it.
This is especially interesting since it shows a different mode of transmission from the 'Pengoloð - Ælfwine' tradition: the Annals are conceived as a written work made in Númenor, deriving from the 'Exiles', the Noldor in Middle-earth, who themselves derived it from the work of Rúmil.

So we have the evolution in this way: Rúmil was the author of the Annals of Valinor, and the later dates were made by Pengoloð, in the next version, the rôle of Pengoloð is discarded from it, and the whole work is attributed to Rúmil. Christopher Tolkien had a theory that Rúmil was a ñoldorian elf of the house of Finarfin, and returned with him to Valinor. That theory had it’s argument in the fact that in the first version of the Annals of Valinor, the work of Rúmil stop with the uttering of the Doom of Mandos and the returning of Finarfin to Aman. This unfortunately is highly unlikely because in the next version, Rúmil is the sole author of it. It is interesting also to note the change of the transmission of the tales in Middle-Earth. First he had the conception of the sailor Ælfwine, who came to Tol-Eressëa and there learned from Pengoloð the stories. Then as the legendarium evolved, Ælfwine was discarded, and some stories were passed directly from Rúmil to the exiles and from the exiles to the Numenóreans.

There are other traditions which are not written down such as this:
From The War of the Jewels: Quendi and Eldar

’But without argument we know that they did so; for there are references to the Lambe Valarinwa in old lore and histories, though these are few and scattered. Most of these references appear to be derived, by tradition of mouth, from “the Sayings of Rúmil” (I Equessi Rúmilo), the ancient sage of Tirion, concerning the early days of the Eldar in Aman and their first dealings with the Valar.

An interesting point to make is that, the Valaquenta is a work which has no author. I wonder if the The Valaquenta could be ascribed to Rúmil. We can’t know for sure but if I had to bet, I would go with Rúmil.

[Maedhros]

So, who is Rúmil then?
From The Peoples of Middle-Earth: The Shibboleth of Fëanor

Note 30:
the Ñoldor had been from the earliest times most eminent in and concerned with this kind of 'wisdom'; ňolmë a department of wisdom (science etc.); Ingolë (ňgōlē) Science/Philosophy as a whole; ňolmo a wise person; ingólemo one with very great knowledge, a 'wizard'. This last word was however archaic and applied only to great sages of the Eldar in Valinor (such as Rúmil). The wizards of the Third Age - emissaries from the Valar - were called Istari 'those who know'.

Rúmil is an ingólemo of Tirion.

[Embladyne]

Well, Rumil in the Lost Tales has a different character than in the Silm....heh heh, he doesn't even appear in person in the Silm. In the Lost Tales, however, he's a sage of all earthly languages...

Rumil to Eriol: "...Know you that the Nodoli [Noldor] grow astounding slow, and yet have I grey hairs in the study of all the tongues of the valr and of Eldar. Long ere the fall of Gondolin, good sir, I lightened my thralldom under Melko [Melkor, Morgoth] in learning the speech of all monsters and goblins - have I not conned even the speeches of beasts, disdaining not the thin voices of the voles and mice? - have I not cadged a stupid tune or two to him of the speechless beetles?..."

And it continues on about other languages. Obviously, Tolkien changed many things in his mythologies, but at the least, this shows us the beginnings of Rumil, and gives insite into how Tolkien may have come to see him.