what kind of helmets and useful of this character-thief/mage!!!!

Can't use a helmet...BUT...

SPOILER ALERT

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...be on the lookout for those cool Ioun stones! :D Can be worn as a helmet, and give some neat effects as well.

-Sazerac

oh....ok!! thanks!!!

SPOLIER

Also if u have T-O-B and u kept the Bladesinger chain from
that Black Dragon in the elves place u can give the Bladesinger
chain and a protection from normal weapons scroll to cespenar the imp
and he will make some sweet armor with immuntiy to normal weapons. A good helmet is the theives hood . .

you need the theives hood from a extraplanar dude in the 3rd level
of the watchers keep. . an antidote potion and a ring of invisibilty
give that all to cespenar and he will make a NEW theives hood
with the wearer immune to backstab and poison damage can also cast true sight 3 times a day

Ioun stones don't work for my mage, or my thief. The game considers the stones to be helmets. Is this a bug?

Yes, Ioun stones are the best. Monks can use these also, so be on alert, one is in the Temple Ruins just for info.

Not all Ioun Stones are wearable by a Mage or a Thief. The best Ioun Stone for a Mage will have to be the Pale Green Ioun Stone.