The mirror-universe Julian Bashir is a scruffy, quarrelsome member of the Terran rebellion, a fighter rather than a physician, who resents taking orders and frequently clashes with his own comrades.
In the mirror universe Julian Bashir has none of the polish of the Starfleet doctor. He is a rough and aggressive rebel who has grown up under Alliance rule, and he is quick to challenge anyone who claims authority over him.
He first appears in Through the Looking Glass, among the rebels hiding in the Badlands. He is suspicious of the man he takes to be his returned captain, and the prime-universe Sisko has to knock him down to keep up the pretence of being the rebel leader.
Bashir returns in Shattered Mirror, taking part in the defence of the captured Terok Nor, and in The Emperor's New Cloak.