The Genie acts as a servant to the Sorcerer who has captured him, doing his bidding if its trickster mind can be overcome.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
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A Genie is not deployed on the table at the start of the battle, but may instead be summoned at the start of any of your turns (as long as they are not already summoned and alive) by the Sorcerer who controls the Genie by passing a Leadership test (using their own Leadership characteristics). Place the Genie within 6" of the Sorcerer, at least 1" away from other units or impassable terrain. It may act normally in the turn it is summoned. If the Genie is slain, it can be summoned back in a later turn, but the Sorcerer will suffer a cumulative -1 penalty to the Leadership test required every time they try to do so. However, if the Sorcerer is slain, the Genie is immediately banished back to its container and removed as a casualty.