Mercy Graves was a tough young woman with a checkered past, who served as Lex Luthor's personal bodyguard and chauffeur.
Little is known about Mercy's life before working under Lex Luthor, except that she lived in the street until he took her in, cleaned her up, and made her his right-hand woman.
Mercy was loyal and respectful to Luthor, but never servile. There are some hints that she was attracted to Luthor, and fiercely resented any woman who came between them. When Luthor became unusually taken with Lana Lang, Mercy secretly followed her and discovered her passing information on Lex's activities to Superman. Mercy was proud of what she'd done, though Luthor took out his disappointment on her.
When Luthor was abducted by Brainiac, she once teamed up with Superman to find him. Superman was baffled by her loyalty to Luthor, initially believing that she had some dirty secret that Lex was threatening her with, and taken aback to hear her say she served Luthor willingly.
Superman tried to convince her that Luthor did not return her loyalty, but she refused to listen.
In the ensuing battle with Brainiac, Mercy was pinned under a pile of fallen machinery while the room was caving in. She called for Luthor to help her, but he chose to save his own skin, leaving her to die. Instead, she was saved by Superman. Afterwards, she continued to serve Luthor, probably trying to forget his betrayal.
Becoming CEO of LexCorp
When Luthor went to prison, he appointed Mercy as acting head of LexCorp. She traded in her old driver's costume for a classy skirt suit, and grew out her hair. Somewhat to her own surprise, she thrived: not only did she save the company from the slump caused by Luthor's arrest, she made it more powerful and profitable than it had ever been. When Luthor, a fugitive from the Justice League, appeared in her (formerly his) office pleading for help, she coldly rejected him. According to her, she had "figured out" that the only reason he left LexCorp in her care was because he thought of her as the only one so slavishly loyal to him that she'd hand it back when asked. Given this unflattering assessment of her character, and her undeniably excellent performance as a CEO, she said she no longer had to lick his boots. Luthor responded with violence, and she reluctantly guided him to Professor Ivo's residence, as he was seeking a cure for his Kryptonite poisoning.
Although the Justice League was searching for Luthor, she concealed his whereabouts and continued to aid him, albeit reluctantly. Even as she did so, he taunted her, saying she couldn't resist the attractions of his power and intelligence (implying that at some point in the past their relationship was more than platonic). However, after Luthor was humiliated by the android Amazo and sent back to prison, he called her on the phone, demanding that she obtain lawyers and medical experts for him.
She just hung up on him coldly. 
Later, however, after he was released from prison and received a full pardon from the President, Luthor regained control of LexCorp, and Mercy returned to his right side, implying that her newfound self-confidence had gone.
Powers and abilities
Though she usually relies on her cool and sardonic wit as her first form of defense, Mercy can be very physical and is an absolute demon in a fight. Despite the impracticalities of her uniform's short skirt, she uses a rough, street-form of kickboxing as her preferred form of attack, and can take anyone this side of Superman in one-on-one combat. She's also an expert with most forms of handheld weapons and has no problem with using them when she has to. However, she is no match for advanced fighters such as Batman who easily decked her with a single punch.
Mercy was one of a small number of characters originally created for the DCAU who was later adapted into DC Comics. She first appeared in Detective Comics #735 (part of Batman: No Man's Land), was in a situation with Batman and Lex in Batman #573 similar to that in World's Finest, and later became Luthor's personal bodyguard (along with a woman named Hope) after his election as President of the United States.
In the comics continuity, there is a suggestion that Mercy and Hope are Amazons, as they have sufficient strength to stagger even Superman, and recognize the sorceress Circe on sight, just as Wonder Woman does. Both Hope and Mercy have left Luthor, and both seemed to be interested in working with Steel against their former boss.
A small tribute to her similarity with Harley Quinn is made in "World's Finest", when the pairing off of Luthor and Joker as antagonists leads the two "hench-wenches" to face off, and engage in a brutal fight that goes apparently unnoticed by their employers. (They fight again when Joker shows up in Metropolis in Action Comic #765.)
Interestingly, Harley disguised herself as Charlie's limo driver in Batman: The Animated Series episode "Joker's Favor", which makes her resemble Mercy. Of course, this is four years before Mercy first appears in Superman: The Animated Series. In a similar fashion, Harley Quinn disguised herself as Mercy during the World's Finest three parter when she first encountered Lex Luthor, having knocked Mercy out earlier with a mechanized punching glove.
In the eighth season of the television series Smallville, the character named "Tess Mercer" is a fusion of the names Eve Teschmacher (from Superman: The Movie and Superman II) and Mercy. Much like Mercy, she is blindly devoted to Luthor and carries out his every command, claiming he saved her life a few years ago. In the episode "Toxic", Oliver Queen gives her the nickname "Mercy". In the final season however, it was revealed she is Lex Luthor's half-sister, Lutessa Luthor.
- "A Little Piece of Home"
- "Stolen Memories" (Cameo)
- "The Main Man, Part I" (Cameo)
- "My Girl
- "Blasts From the Past, Part I" (Cameo)
- "Livewire" (Cameo)
- "Identity Crisis
- "Solar Power" (Cameo)
- "Brave New Metropolis" (alternate universe)
- "Ghost in the Machine"
- "World's Finest"
- "Apokolips... Now!, Part II" (Cameo)
- "A Fish Story" (Cameo)
- "Legacy, Part I"
- Bader, Hilary J. (writer) & Tomonaga, Kazuhide (director) (September 14, 1996). "A Little Piece of Home". Superman: The Animated Series. Season 1. Episode 5 (airdate). Episode 5 (production). Kids WB!.
- Dyer, Sarah (writer) & Aoyama, Hiroyuki (director) (September 29, 1997). "Ghost in the Machine". Superman: The Animated Series. Season 2. Episode 14 (airdate). Episode 15 (production). Kids WB!.
- Bader, Hilary J. (writer) & Yano, Yuichiro (director) (November 23, 1996). "My Girl". Superman: The Animated Series. Season 1. Episode 11 (airdate). Episode 10 (production). Kids WB!.
- Berkowitz, Stan (writer) & Riba, Dan (director) (October 4, 2003). "Tabula Rasa, Part I". Justice League. Season 2. Episode 3 (airdate). Episode 29 (production). Cartoon Network.
- McDuffie, Dwayne (story) & DeMatteis, J.M. (teleplay) & Riba, Dan (director) (June 11, 2005). "Clash". Justice League Unlimited. Season 1. Episode 20 (airdate). Episode 20 (production). Cartoon Network.