It is important to bear in mind that magical forces that brought the "gang" to Mystara, along with the extremely potent incantations of their "totem" items give the heroes their abilities. Class and level are assigned to make sense of saving throws, hit points, and so on, but in some ways, these guys just don't fit into the game as proper examples of their class. It's best to just treat them as unique creatures and not worry too much about tweaking them to adhere completely to the rules.
Sheila the Thief
7th Lvl Thief, Lawful
Aside from her +2 Leather Armor and the Cloak of Undetectability, Sheila carries the usual rations, supplies and gear for a thief type adventurer.
Sheila is perhaps the least likely among the "gang" to choose a life of crime, which may be the exact reason the DM chose her to bear the Cloak of Undetectability, seeing that she was least apt to abuse its power for greed or evil. Sheila's an honest, caring girl, more than a little frightened by the living nightmare her life has become, and always afraid of losing her friends and being alone.
She makes great efforts to hide her fears and protect and be a good role model to Bobby, who is her little brother. She's non-confrontational by nature, and eager to fit into social groups and be liked, so she tends to go along with her friends rather than figure out her own path, but she isn't quite as gullible as she seems. Her insecurity, not any lack of brains, is her biggest weakness.
The Cloak of Undetectability (Totem of the Thief; Minor Artifact)
Woven from an unknown, shimmering purple fabric, the cloak appears well constructed, but shows no signs, even to a Detect Magic spell, of being enchanted. This applies to mortal magics like Analyze and the Slate of Identification as well, these will never reveal any information on the cloak.
When the hood is put up and the cloak pulled around the body, the wearer becomes Invisible (as the Magic User spell). The wearer may move and make sounds normally without interfering with the cloak's magic, but if she engages in combat (other than to dodge and flee), including being "accidentally" hit by stray fire in combat, the wearer becomes visible and may not activate the cloak again for 1d4 rounds.
In addition, once per day the wearer may invoke the cloak's real power, becoming not only invisible, but also Silent (as the Magic User spell Silence 15' Radius, but applying only to the wearer, no radius effect) and undetectable by any mortal sense other than touch, and any mortal scrying magic. This lasts for 1d6 turns, subject to the same caveats for combat participation as the normal invisibility power is. The wearer may of course also discontinue the power at any time by taking the hood off.
When worn by a normal human or demi-human (0 level, no class), the cloak imparts the HP, Saving Throws, "Thieving Skills" and Combat Abilities of a 7th level thief. This boon does not apply to classed characters.
Anyone stealing the cloak, or harming or killing Sheila to possess it will instantly draw the enmity of her remaining companions (and the Dungeon Master, who will indirectly aid them in avenging their comrade) as well as the ire of Venger (who will be detailed in a separate post here soon). It's not apparent to the heroes, but Venger fears the totems, not necessarily their owners, as a threat to his and Tiamat's power. New owners, potentially much more aware of their world and more of a threat to Venger and his evil mistress, will only alarm him more.