Loki was known as the OG "Battle Rapper," but who was he to Vali and Vidar ?
Vidar and Vali are two of the most under discussed characters of the Eddas, but that's just my opinion.
The Eddas both Prose, and Poetic, have very important details in the stories held within them. Many discussions can be found on the more popular poems like Havamal, and Voluspa. There are not very many detailed explanations, or debates, that exist on Lokasenna, or even Gylfaginning.
The Original Battle Rap
While Loki is Flyting (Battle Rapping) with a few of the gathered Æsir gods in Ægir’s Hall. They are in the middle of their feast, when Loki barges in, and he demands a plate of food, and a seat at the table. His relationship with Odin, or at least part of it is explained in this story. This is the “Henry Adams Bellows” 1936 translated version, so all of the “d’s” will be replaced with a “th” other than that minor detail, it's fairly easy to follow.
Lokasenna : 8 - 16
Bragi spake:
8. "A place and a seat | will the gods prepare
No more in their midst for thee;
For the gods know well | what men they wish
To find at their mighty feasts."
Loki spake:
9. "Remember, Othin, | in olden days
That we both our blood have mixed;
Then didst thou promise | no ale to pour,
Unless it were brought for us both."
Othin spake:
10. "Stand forth then, Vithar, | and let the wolf's father find a seat at our feast;
Lest evil should Loki | speak aloud
Here within Ægir's hall."
Then Vithar arose and poured drink for Loki; but before he drank he spoke to the gods:
11. "Hail to you, gods! | ye goddesses, hail!
Hail to the holy throng!
Save for the god | who yonder sits,
Bragi there on the bench."
Bragi spake:
12. "A horse and a sword | from my hoard will I give, and a ring gives Bragi to boot,
That hatred thou makst not | among the gods;
So rouse not the great ones to wrath."
Loki spake:
13. "In horses and rings | thou shalt never be rich, Bragi, but both shalt thou lack;
Of the gods and elves | here together met
Least brave in battle art thou,
(And shyest thou art of the shot.)"
Bragi spake:
14. "Now were I without | as I am within,
and here in Ægir's hall,
Thine head would I bear | in mine hands away,
And pay thee the price of thy lies."
Loki spake:
15. "In thy seat art thou bold, | not so are thy deeds, Bragi, adorner of benches!
Go out and fight | if angered thou feelest,
No hero such forethought has."
Ithun spake:
16. "Well, prithee, Bragi, | his kinship weigh,
Since chosen as wish-son he was;
And speak not to Loki | such words of spite
Here within Ægir's hall."
Within these 8 stanzas, a great deal of information is given here. First, we see that Loki, and Odin, are blood oath sworn brothers, in the sense that, any table where Odin is welcomed, so to must Loki be welcomed. We can also see that Odin himself honors his oath.
What happens next, is Vidar’s reply to Bragi regarding him denying Loki a seat, it is what appears to be a small detail. It may be far from that though, especially in the story of who Vidar is. By most accounts in the Eddas, Vidar is named as one of Odin’s many sons. His defense of Loki here seems innocuous, but is it really something far deeper?
Along with Thor, Baldur, Hodr, Vali, and a few others, Vidar is one of the All Fathers offspring among the Æsir and other Gods who fight in Ragnarok. In the battle, Vidar defeats the Wolf Fenrir, but only after Odin is killed. Vidar makes it out alive and is one of the 8 survivors. These tales are commonly known, even by many who are not familiar with the Eddas themselves, but rather from pop-culture. Many who are fairly well versed in “Odinism” may not even be aware there are stanzas that may indicate, that Vali, the brother of Vidar, was Loki's son, and Odin may have just taken him as a ward after the killing of Baldur.
Let's dig into that.
Who Is Vali ?
In this portion of “ Gylfaginning : Stanza 50 ” We start where Thor has Just Captured the fish that Loki had transformed to in a net. They were hunting for him after he caused the death of Baldur. We see that Odin gets part of his justice on Loki, by making him watch as his son Vali, kills his other son Narfi. The way in which it happened was horrifying, first Odin changed Vali into a wolf, then he devoured his brother Narfi, while Loki was forced to watch. Then Loki is actually bound with the entrails of Narfi.
But don't take my word for it, read it for yourself below in its full gory form.
Gylfaginning : 50 (excerpt)
Thor clutched at him and got hold of him, and he slipped in Thor's hand, so that the hand stopped at the tail; and for this reason the salmon has a tapering back.
"Now Loki was taken truceless, and was brought with them into a certain cave. Thereupon they took three flat stones, and set them on edge and drilled a hole in each stone. Then were taken Loki's sons, Vili and Nari or Narfi; the Æsir changed Váli into the form of a wolf, and he tore asunder Narfi his brother. And the Æsir took his entrails and bound Loki with them over the three stones: one stands under his shoulders, the second under his loins, the third under his boughs; and those bonds were turned to iron. Then Skadi took a venomous serpent and fastened it up over him, so that the venom should drip from the serpent into his face. But Sigyn, his wife, stands near him and holds a basin under the venom-drops; and when the basin is full, she goes and pours out the venom, but in the meantime the venom drips into his face.
There is not an Eddic source that I have come across yet, saying directly whether or not both Gods may have had a son named Vali. However, given the other insinuations made, it's easy to determine that this is the case. Again this is mentioned later in Lokasenna, as Loki and the Gods are still Flyting in Ægir’s hall.
Lokasenna : Stanzas 63 - 65
Thor spake:
63. "Unmanly one, cease, | or the mighty hammer,
Mjollnir, shall close thy mouth;
The slayer of Hrungnir | shall send thee to hell,
And down to the gate of death."
Loki spake:
64. "'1 have said to the gods | and the sons of the god,
The things that whetted my thoughts;
But before thee alone | do I now go forth,
For thou fightest well, I ween.
65. "Ale hast thou brewed, | but, Ægir, now
Such feasts shalt thou make no more;
O'er all that thou hast | which is here within
Shall play the flickering flames,
(And thy back shall be burnt with fire.)"
And after that Loki hid himself in Franang's waterfall in the guise of a salmon, and there the gods took him. He was bound with the bowels of his son Narfi, but his son Vali was changed to a wolf. Skathi took a poison-snake and fastened it up over Loki's face, and the poison dropped thereon. Sigyn, Loki's wife, sat there and held a shell under the poison, but when the shell was full she bore away the poison, and meanwhile the poison dropped on Loki. Then he struggled so hard that the whole earth shook therewith; and now that is called an earthquake.

Changed To A Wolf.
That phrase is used more than once in the Eddas. Does it imply that Vali was physically changed into the form of a wolf? Or does it instead say that he became an Ulfhednar. For those who are unfamiliar, the Ulfhednar were the elite (Wolf headed) soldiers of Odin. They were a well educated and trained tactical unit. They traveled, lived, and hunted together as a unit.
To me the stanzas above would imply that Vali was adopted by Odin, or taken as a ward. He was then trained as an Ulfhednar. His first mission, was to kill one of the participants in Odin’s favorite son Baldur’s death. His target was another son of Odin named Hodr. This also explains how Vali on his first day as the son of Odin began this mission. Perhaps he was not a newborn, and he was the same “wolf” teenage ward that Odin took from Loki.
Skaldskaparmal 13 :
How should one periphrase Hödr? Thus: by calling him the Blind God, Baldr's Slayer, Thrower of the Mistletoe, Son of Odin, Companion of Hel, Foe of Váli.
What do you think?