Minion: Protect the Boss: Whenever a friendly character named Lex Luthor is within 8 squares, modify Mercy Graves' attack and speed values by +1 and she can use Defend.

See Minion powers for common questions.

Since Lex Luthor activates this minion power, does that mean that Mercy may only use Defend on Lex?
No. So long as this power is activated, she can use Defend normally. Any character on her team can potentially benefit.