Shir is a somewhat unconventional thief character for a JRPG in that she does not have any stealing techniques during battle; rather, she has a small chance to steal items from shops whenever you visit. When she does, she will disappear from your party and only appear again once you revisit Rolf's house.
If you bring her to level 10 and visit the storage room in Paseo's central tower, she also has a small chance of stealing the Visiphone, an extremely useful item that allows you to save your game anywhere, anytime. It's well worth the trouble.
Stuff Shir can steal
There are a few stealable items that you can't buy in any store. These are marked with an asterisk.
*Fire Staff (Best weapon for Amy, casts Foi when used in battle)
*Jewel Crown (Best headgear for Anna and Shir)
*Green Sleeves (Shield for Anna, casts Shinb when used in battle)
*Star Mist (Fully heals the entire party)
*Moon Dew (Revives a dead character to full HP)
As you'd probably expect, the odds are tilted to give you the crap items over the good ones. Don't be surprised if you get about 5-10 Daggers and Laser Knives for every Fire Staff you find...