let me help you with that.
Spirits (unless otherwise stated by a spell, or unless they are a knight), are intangible. they do not eat or sleep, being either the embodiment of the holder's own "ideals" or an actual spirit (I never quite figured out which). Other than knight spirits or in special circumstances (like what happened with Medusa and Sophia in Will O' the Piper), a spirit cannot materialize at all. Unless otherwise stated by a spell, they are bound to their own form.
Those are the answers to your questions, based on my own knowledge. ( DISCLAIMER: The author of this comment does not make any claims to the validity of this statement, though he believes the information to be accurate enough)