Mary Shelley left a lot of the monster out of her Romantic novel Frankenstein. Given that Shelley desired her readers to be as frightened as she herself had been frightened.
O! if I could only contrive one which would frighten my reader as I myself had been frightened that night! (1831 Introduction to the novel)
Therefore, Mary Shelley left out a detailed description of what the monster looked like in order to allow readers to create a picture of the monster on their own. The only image Shelley offers readers is that of the monster's yellow eyes. Outside of that, readers only know the general size ("being of a gigantic stature; that is to say, about eight feet in height") and that Victor intended the monster to be beautiful.
His limbs were in proportion, and I had selected his features as beautiful. Beautiful!—Great God!
Intellectually, the monster is very advanced. The fact that he was able to learn language through listening shows him to be very intelligent. Not only that, the monster is able to read.
Overall, the monster is quite hideous (although this is inferred) and very intelligent (shown through his ability to understand complicated texts--such as Paradise Lost, Plutarch's Lives, and the Sorrows of Werter. The monster also shows the ability to love (as determined through his desire to love another) and show compassion (seen in his rescue of the drowning girl).