Tweek is an Expiremental Fairimental made by the Fairimentals to stay on Earth for a long period of time. He becomes a part of the gang, but is known that in bad situations, he spontaneously implodes in a poof of twigs and shrubbery.
Tweek is an awkward, nerdy fairimental. He is an E.F. (Expiremental Fairimental), the first of his kind. He first appears in Song of the Unicorns, where he develops a good relationship with the mages.
Tweek had a big part in Song of the Unicorns, though he did not really have a big part in any other books, though he did appear in Avalon: The Warlock Diaries.
Tweek has a special jewel called the HORARFF (apparently there is a HORARFM for mages, but the Mages never discovered this), it stores information, unlike mage jewels, which harness magic. HORARFF stands for Handbook of Rules and Regulations for Fairimentals. He is checking and using it nonstop.
Ozzie and Tweek are good friends, both being companions and mentors to the mages. The Mages are Tweek's best friends, they keep him from exploding into twigs (and sometimes they do not do a very good job).