"Never mistake a woman's meekness for weakness."
- Serena Joy in her book about "domestic feminism"
Serena Joy is a major character in The Handmaid's Tale. She is the Wife of Commander Fred, who is using Offred to try and obtain a child, being unable herself to conceive. In the film adaptation, she is portrayed by Faye Dunaway, and in the television series, she is portrayed by Yvonne Strahovski.
In the novel Edit
Offred states that Serena Joy's real name is Pam, and that Serena Joy is a stage name. Before the creation of Gilead, Serena was a televangelist and talented singer, who advocated for women to return to 'traditional' family values and roles and supported Gilead's creation. At some point, she married a man named Fred, who became a high-ranking Commander in Gilead. However, after Gilead's triumph, Serena was left bitter and miserable. She was stuck at home all day and had to rely on Handmaids to conceive a child.
Serena Joy is apparently aware that the Commander breaks rules in regards to keeping contraband items, but turns a blind eye out of love for him. She herself smokes cigarettes, a banned item. What she does not tolerate though, is her husband becoming too familiar with their Handmaids. When Serena found out that Offred's predecessor had been visiting her husband behind her back, she was furious. It is implied that the Handmaid committed suicide partly out of fear as to what Serena would do. Serena makes it plain that she doesn't like Offred, but views her as her only chance to gain a child. She secretly strikes a deal with Offred, arranging for her to meet with Nick in private - whom she trusts - so as to conceive, stating that many Wives do this. She offers Offred cigarettes and information on her daughter - whom Offred has not seen for years - in exchange.
When Serena learns that Offred has been conducting an emotional affair with the Commander, after finding lipstick stains on her blue shawl (which Offred had worn as a disguise when she visited Jezebel's with the Commander), Serena Joy angrily confronts her, asking how she could betray her like this after she tried to help her. When Offred is arrested by the Eyes, it becomes apparent that Serena Joy did not turn her in in revenge, as she looks horrified and calls Offred a "bitch", apparently believing Offred had reported Fred to the Eyes in either revenge or an attempt to save herself.
The epilogue discusses Serena Joy's real identity; the most likely candidate for her husband is Commander Fred Waterford, however, his wife's name was Thelma, not Pam; the other most likely candidate did not have a wife named Pam either. This would suggest that Offred misremembered Serena's real name, that she is a completely different person or even that Offred used a false name for some reason; using Serena's real name could've identified Offred, endangering her loved ones, or it's possible that, despite Serena's generally horrible treatment of her, Offred was attempting to protect Serena herself, as she would likely have gotten into a great deal of trouble for helping Offred commit adultery with Nick.
In the Hulu series Edit
Serena Joy is portrayed by Yvonne Strahovski. In this adaptation, Serena Joy seems to be portrayed as being younger (in her thirties) than she is in the novel, where she is implied to be middle aged, though she is still apparently infertile.
Before Gilead Edit
Serena is a conservative cultural activist, with a passion and intelligence equal to that of her husband (as well as a special fondness for movie popcorn). According to Castillo, she was once even arrested for inciting to riot, and she wrote a book about her beliefs, titled "A Woman's Place", which contains the famous line "Never mistake a woman's meekness for weakness". She is considering to write another book about "fertility as a national resource" and "reproduction as a moral imperative".
Her husband is one of the conspirators who devise the (alleged) "terrorist" attacks. Serena is confident that the staged attacks will help quickly propel the envisioned society that is conducive to her own theocratic beliefs.
After the takeover Edit
Serena is shown to be completely shut out of the new government planning since the new society won't let women "forget their real purpose" again, as Commander Warren points out to a dismayed Fred. Rather symbolically, as she prepares to accept her new limited role in the society she helped create, a copy of her book is seen being thrown out in the trash.
Present time Edit
Serena Joy takes Offred to the Putnams to see Ofwarren and the baby. Ofwarren tells Offred that her Commander is in love with her and that they're going to run away. She's also given her daughter a different name, "Charlotte". Offred fears Ofwarren is delusional and comments on her behavior to Serena Joy. Nick drives Offred home and there is a black van waiting. Offred is interrogated by an Eye and Aunt Lydia about her knowledge of Ofglen. Serena intervenes yelling at them to stop beating her because Offred is pregnant. Offred later reveals to Serena that she's not pregnant, causing Serena to drag her to her room and order her not to come out. 
Serena Joy calls Offred outside to tell her she's asked Nick to try and impregnate Offred. She suspects the Commander is infertile, so she secretly brings Offred to Nick's apartment, where she waits as they have sex. They finish, and Serena tells Offred to lie down in her room, after saying a prayer to encourage her to become pregnant. 
The Waterfords receive a Mexican delegation being lead by Mrs. Castillo, who is intrigued by the idea of Handmaids. She asks Serena if she ever imagined a society in which women can no longer read her book, to which Serena replies that God asks for sacrifices.
Serena wakes up Offred to rush to help Janine (formerly Ofwarren), who is threatening to jump off a bridge with her daughter to be free. Offred convinces her to release the baby, which Janine does right before she jumps off. Commander Warren is arrested. Serena tries to offer comfort to his wife Naomi and offers help with the baby, which she curtly refuses. Naomi then reminds Serena of the first Offred, saying that "Men don't change". 
Serena discovers that Offred went to the Jezebel's club with her husband and strikes Offred down on the ground to punish her. She then forces Offred to take a pregnancy test, which is positive. Serena reveals the good news to her husband and tells him the baby isn't even his because he is weak. She then takes Offred on a surprise trip, several hours away. Serena meets with Hannah, Offred's daughter, to show Offred that if she harms herself or the baby then Hannah will be in danger. Offred screams in misery and calls Serena an evil monster. 
Two armed guards come in and lead Offred away, to the surprise and horror of Serena Joy and Fred. Offred walks off willingly, unsure if this is her end or a brand new beginning.