As I have commented several times before in previous articles *Tiwaz was originally the supreme deity of the Germanic peoples, to be usurped later by *Thunaraz and then *Wodanaz. Beyond the few stories within the Eddas there is little that we know about this God. However Tacitus has left us a clue in his Germania:
"In ancient lays, their only type of historical tradition, they celebrate Tuisto, a god brought forth from the earth. They attribute to him a son, Mannus, the source and founder of their people, and to Mannus three sons, from whose names those nearest the Ocean are called Ingvaeones, those in the middle Herminones, and the rest Istvaeones. Some people, inasmuch as antiquity gives free reign to speculation, maintain that there were more sons born from the god and hence more tribal designations- Marsi, Gambrivii, Suebi, and Vandilii- and that those names are genuine and ancient." (2.2)
Many scholars, such as Rudolf Simek associate Tuisto with Ymir:
"Tuisto seems to be related to the proto-giant> Ymir as the earth-born hermaphrodite ancestor of the race of man, and therefore the meaning of the name Tuisto as `hermaphrodite` is quite likely. The form of the name Tuisco (the actual manuscript reading) can also be traced back etymologically to the same basic meaning." (Dictionary of Northern Mythology)
`Tuisto` can be connected to the Proto-Germanic root word *tvai which means `two`. The variant `Tuisco` may be reconstructed as *tiwisco and thus may be linked to *Tiwaz. John Lindow in his Handbook of Norse Mythology tends to be dismissive of the connection between Ymir and Tuisto as the former is seen as a negative entity whilst the latter was worshipped as a God and in this point I agree with him. Jacob Grimm in his Teutonic Mythology Volume 1 makes a very clear association between *Tiwaz and Tuisto:
"If the earthborn Tuisco, the ancestral god of our nation, stands (as Zeuss p.72 has acutely suggested) for Tivisco, Tiusco, it shews on its very face the meaning of a divine heavenly being, leaving it an open question whether we will choose to understand it of Wuotan or any other god, barring always Tius himself, from whom it is derived."
*Tiwaz can be traced back to the Proto-Indo-European *deiwos, meaning `celestial being` and cognates of this deity may be found amongst other Indo-European peoples such as the Celts, Lithuanians, Latins and Indo-Aryans. This Sky Father was there at the very beginning of the Aryan and then later Germanic peoples. Even the later *Thunaraz and *Wodanaz shared certain aspects of His role as the Sky Father, the All Father.