You may be as minimalist or detailed as you want. The avatar does not have to be humanoid. Here is an avatar I made:
The Professor: a very tall, vaguely human-like creature. He wears a plaid suit of humble green. His vertebrae are scrolls which unroll their long paper in many directions, making it seem as if he has tentacles coming out of his back. The paper is covered in living equations, illustrations, poems, music scores and other intellectual works from different realms that alternate between two and three-dimensional. His hair is white and modest. He has eyes all around his body, which seem to be both dark like academic ink and bright like the light bulb of inspiration. His skin alternates between leathery, stone and metallic. His hands hold a quill pen of everlasting ink and a tome of never-ending pages. His teeth are chalk, his tongue is black ink and a small cloud of dust hovers around him. His voice is wisdom and intellect, incommensurate and immemorial. A minute of conversation makes even those who are full of hubris ponder at what they really know. He grants valuable occult lore to those who go through great danger in major service of the Library. In his mind, lies are a corruption of truth. Truth and Knowledge are twins. Thus, if you lie, you are perverting Knowledge, which is the Essence of the Library. That does not please him.