The Seer is the Oracle of Kattegat. Seers are mystical soothsayers with supernatural powers who can communicate with the gods & answer questions about the future and one's fate in life. Vikings believe that their fate is already decided, and turn to seers to translate the gods' wishes and intentions.
His past is as obscure as his own character. It is not known if his deformity is congenital, by disease or even violence - neither can we be sure if he even has eyesight. His shamanic abilities (including visions), however, can be confirmed - which probably makes him one of the most revered characters in Kattegat. Enjoying a status oftentimes more momentous than the king himself, he is seen as straddling two worlds, in contact with the gods. These rarest of abilities make him not only honored & valued but also essential.
He may actually be one of the gods but in human form ( just like Harbard). In one of the episodes of season two The Seer tells Ragnar a story about a marriage of god Njord and a giant Skadi. He described Njord as a god of the sea, with an old and withered skin and large but smooth feet. In the same scene we can see the entire body of The Seer and his bare feet. That scene was a clue and The Seer was probalby talking about himself. In many episodes he also admitts that he has lived hundreds of years. He may not be actually alive or dead, but something in between. We know that his gift is also his curse.
The Seer is devoted to the Norse gods, and has a gruff, impertinent, meek though wise persona. He also seems very aged, and possibly with great direct life wisdom/experience as much wisdom from the gods. Like Floki, he sees other religions as a threat to his religion. When Earl Haraldson reveals his doubts about the existence of said gods, the Seer laughs at him as if he is a fool. The Seer is tormented by his visions, once telling Lagertha that he does not wish to relate her future since he sees only sorrow. ("All Change") The seer also tends to think and speak in laconisms. This indicates a strong visual mind (much like Floki) but overly-active nerves - strangely so, for our mystic is in fact so inert that he resides in a state of self-imposed hermeticism - in itself a horrible condition, as opposed to the hustle-bustle environment more often expressed in Vikings. Unlike Floki, however, he does not seem overtly intolerant of other religions (particularly Christianity). He never expresses his feelings (fear, happiness, ambition or preoccupation). Neither does he show any political view or affiliation; he is seen giving counsel to, among others, Ragnar, Earl Haraldson, Jarl Borg, Rollo, Lagertha and Aslaug - forewarning one against the other. He also shows little concern regarding the apparition of Harbard, which is conceivably a manifestation of the Gods. He does, however, suffer for his art and we are shown him suffering in one of his related dreams.
Many of the Seer's prophecies are deliberately ambiguous, as he himself has said that the prophecy must only be fully realized once it's too late to change it. He also has said that humanity cannot handle too much of reality, and this also may permit some degree of free will or contemplation in the recipient of his counsel. For instance, he told Rollo about his impending Dukedom in Frankia, though in the manner that if he knew what was in store he "would dance naked on the beach", and that "the Bear will marry a Princess and he will be at the ceremony". Moreover, he told Ragnar that whilst the dead but not the living would conquer Paris, the Bear would marry a Princess and it would not bode well for him. This intrinsic ambiguity allows a variety of interpretations and choices, without compelling the recipient to any firm course of action.
He lives alone. If not for his skills as a shamanic Seidman, he would likely be an outcast. But instead he is one of the most respected characters in Kattegat.
The normal term for a "seer" in Norse society would be a Seidmadhr or Spaemadhr, though it was rare for a man to be practicing 'magic', so this role normally belonged to women called Völvas. In the original script, the Seer was actually a woman.
The reference to 'blind seers' is common in Celtic culture: some sources claim that the irish fildhe (druid-poets) were ritually blinded with hot iron, and blind prophets such as the Greek Tiresias (or even the bard Homer himself) suggest that ritual blinding of prophets and bards was seen as a way of ensuring that their prophetic or poetic powers could be drawn only from spiritual sources.