For one thing, Lady Shiva is a huge character in DC Comics. An assassin-for-hire, she is regarded by many within the DC Comics universe as one of the best living fighters, using her bare hands and finely honed talents to take out her targets. She has appeared in a number of different comics, although she is especially closely associated with Batman, and any Arrow fans knows that the show has... borrowed pretty heavily from Batman stories in the past. In fact, Lady Shiva was the person who taught Batman how to fight again after he had his back broken by Bane. On a show that has already brought in the likes of Deathstroke and Ra's al Ghul, a big character from comic lore would make sense.