Serena

 (codename Sierra and later Elan) is a supporting character in Until Death Do Us Part. She first appears as a member of The Wall, accompanying Alfa when they go to ascertain Haruka Tooyama's precognitive abilities.

After meeting Haruka, Serena grows attached to the girl, who is around the age of her late daughter. She decides to stay with Team Blade to serve as a liaison with The Wall, as well as to keep an eye on Mamoru Hijikata and protect Haruka.

Personality
Serena has a very maternal nature, which results in her quickly becoming attached to and protective of Haruka, who reminds her of her deceased daughter. As the member of The Wall, which hinges around tight coordination among team members, she also values her companions over apprehending criminals.

However, Serena's protective nature can be imposing and, at worst, self-righteous. She obsesses over Haruka and bemoans how the girl "picked" Mamoru over her, despite knowing that Haruka has foreseen Mamoru as the only person who can protect her to the very end. Serena even leaves The Wall to train in the same swordsmanship style Mamoru uses, hoping to better protect Haruka herself; a decision that shocks her ex-husband Kilo and garners ridicule from fellow Wall member Juliet.

History
Several years ago, Serena was happily married to Kilo, with whom she had a daughter named Cindy. However, Cindy was killed by a terrorist bombing and in their heartache, Serena and Kilo joined The Wall. It isn't clear when or why they divorced, but their relationship was evidently strained following their daughter's death.

Haruka Tooyama
Haruka resembles Cindy and is around the same age she would have been, causing Serena to grow extremely attached with her. Several times, Serena tries to separate Haruka from Mamoru, whom she believes to be a bad influence and too violent to keep Haruka around, and eventually even leaves The Wall to learn Mamoru's swordsmanship style so she can protect Haruka herself.

At first, Haruka is less than enthusiastic about Serena's presence, but it is later shown that she is still quite close to the woman. She grows angry when Juliet has less-than-pleasant things to say about Serena, and is excited to see Serena&mdash;as "Elan"&mdash;show up as the last member of Team UDDUP.

Mamoru Hijikata
Serena's view of Mamoru is overall negative. His first interaction with them is to mock them, and while Alfa keeps a cool head, Serena is visibly angry. Over the course of several battles, Serena comes to the conclusion that he is a "danger to us all," and unsuccessfully attempts to separate Haruka from him. Nevertheless, she acknowledges she can't hold to a candle to his abilities, and eventually leaves The Wall to learn his swordsmanship style for herself.

Juliet
Juliet and Serena have a rather tense and stand-offish relationship. Juliet's first statement about Serena is a derisive remark on how the latter "couldn't do her job properly," and she later tells Haruka that Serena's obsession with her dead daughter rubs her the wrong way.

This conflict comes to a head when Serena decides to leave The Wall to learn how to better protect Haruka. Juliet pulls no punches, asking if she was "fucking retarded," then continuing to mock the other woman. After Juliet reveals that her rapist mutilated her and left her unable to conceive, she advises Serena and Kilo to hook back up and have another kid instead of spending their lives obsessing over Cindy.

The two meet each other again as members of Team UDDUP, though the two are able to cooperate well enough.

Kilo
Serena has a complicated relationship with Kilo, her ex-husband. The two are amicable enough to work together on The Wall and are still on speaking terms, but the loss of their daughter greatly affected them.

When Kilo brings up Cindy, Serena stops speaking and turns away from him. After Juliet criticizes their decision to join The Wall in memory of Cindy, and tells Serena to hook back up with Kilo and have another kid, they are left awkwardly together in their hospital room.