Relationship: friend, partner
Name: Selena Kyle
Weight: 133 lbs
First Appearance: Batman #1
History with Steph:
Selina and Steph technically first met in Batman Family #2 where Selina went undercover as a "bodyguard" to a criminal. Though originally she was forced to beat Steph away from her client, the second time Steph went after the man Selina knocked out the perp to save Steph from being shot and allowed Steph to take the crooks to jail.
But Selina didn't recognize Steph's Spoiler costume in War Games, nor did Steph know Selina, so this may have been retconned.
Catwoman helped out Steph during War Games and even revealed her secret identity to Steph.
She even killed Black Mask. Twice. Sure, her reasons for it has nothing to do with Steph, but I say this makes her an ally and more.
Learn more about Selena Kyle here