Alexis is the over-achieving teenage daughter of Richard Castle and granddaughter of Martha Rodgers, with whom she is very close. Her birth mother was her father's first wife, a neurotic actress named Meredith, who Castle refers to as a "deep fried Twinkie."
Her full name is Alexis Harper Castle .
She is the most mature individual in her household, what with her grandmother's flighty personality and her father being in and out of trouble with the police. As she is an intelligent girl, she often provides crucial insight, via a fresh set of eyes or a different perspective, into whatever case Castle is working on with Detective Kate Beckett.
She is so close to her father that he picks her to be Best Man at his wedding to Beckett. 
Alexis Castle is played by actress Molly C. Quinn.
Her first word as a little girl was denouement.
Since Castle was a single parent for most of her childhood, he often took Alexis to the park to play and to the natural history museum where the two would pretend they were on safari for hours on end.
When she was five years old, she and her father began a game of laser tag, which lasted until just before her graduation from high school when Castle scored 1000 points. The game became a tradition between them in which they bonded as father and daughter and also came up with creative ways to ambush each other and score points.
Alexis rebels against the irresponsible nature of her father and grandmother by being well-behaved and remarkably honest. When she tries to keep up with friends in the subway, jumps a turnstile and later lies to her father about the incident, she frets not so much about stealing a ride, but because she lied to her father. In Law and Murder, she again lies to her father about her whereabouts on a day off from school. When he confronts her about why she was dishonest, she explains that some friends had dared her to shoplift some merchandise at a boutique, but she refused and later returned to the shop to leave an amount of cash equal to the amount of stolen goods to compensate the owner.
Even though she is the youngest member of her household, she seems always to teach her father a valuable life lesson, just as much as he tries to do the same for her.
Her personal motto is "Do the thing you fear the most."
Alexis attends Marlowe Prep School (Named after showrunner Andrew Marlowe.)
Another way Alexis rebels is by being a straight A student and involved in several activities in school such as fencing and learning the violin. She even considers acting in her school's production of "Grease", much to Martha's excitement, but gives it up when she realizes she isn't as passionate about it as some of her classmates and that she enjoys the stage crew more. The same happens when she tries out for the school's cheerleading squad.
She begins to show an interest in police work and forensic science after volunteering for a class community service project at the precinct where her Dad works with Detective Beckett. She then does a school project that requires her to study blood splatter patterns. In Pandora, Castle discovers that she has taken an internship with Beckett's and Castle's friend, Medical Examiner Lanie Parish to give her a leg up in case she decides to go to medical school.
As she approaches college, she attends a summer program at Princeton University, then applies for early admission to Stanford University in hopes of attending with her then boyfriend, Ashley, but is deferred. Alexis doesn't deal with it well, and even rings admissions to inform them that they had made a mistake. Having planned on graduating from high school a semester early, she is left with little to do schoolwise during the last half of her senior year. Castle suggests she see the world, perhaps travel, but Alexis chooses to get practical work experience and gets three internships, including the experience with Lanie. Eventually she is accepted to several universities and colleges, including Stanford, but decides to attend Columbia University, so she will be away from home, but not so far that she can't visit if she wants to.
While at university, she becomes involved in a law-school project reinvestigating the cases of people on Death Row. With three days to go before Frank Henson is executed in Pennsylvania, she involves the 12th Precinct and her father in a last-ditch attempt to prove him innocent, which succeeds.
Alexis is asked to her Junior-Senior Prom by a boy named Owen who was in her poetry class at school. She breaks up with him after a few weeks.
She met her first boyfriend, Ashley, when they both attended the summer program at Princeton University. Initially, Alexis worries that Ashley has forgotten about her when she doesn't hear from him after they both return home, but eventually he does call and they start dating. At one point, Castle walks in on them at his loft and catches them kissing.  They date until Cops and Robbers when Alexis breaks up with him, having grown frustrated with his lack of contact with her while she is in New York and he is at college at Stanford.
Once her father gets engaged to Beckett, Alexis goes away to South America for a summer and comes home with a new boyfriend, named Pi. Having lost his passport and since he lives in Amsterdam, he makes himself at home in the loft, much to Castle's chagrin. Further irritating Castle, he is messy, eats only fruit, and has little in the way of boundaries, going so far as to walk into Castle's bedroom wearing nothing but a towel while Castle & Beckett talk in bed. Castle fears that Alexis' relationship with him is dousing her academic ambitions. Further upsetting Castle, Alexis defies her father and moves into an apartment with Pi, but eventually realizes she isn't ready for such a serious commitment. She breaks it off with Pi and moves back into the loft.
Alexis has very little of a relationship with her mother Meredith since she sees her rarely. It is not a negative relationship, but one Alexis tries her best to deal with knowing how crazy her mother can be. Once when Meredith lies to the administrators at her school about a death in the family to get Alexis off for a day of shopping, she protests because of her academic commitments, but eventually goes along with her mother reluctantly.
While sometimes jealous of all the time her father spends at the precinct, Alexis looks up to and respects Detective Beckett. She even seeks out her advice when trying to decide whether or not to study abroad. After Beckett is shot and nearly killed at Captain Roy Montgomery's funeral, Alexis isn't happy with the danger her father subjects himself to by working at the precinct, but eventually she accepts it  and, through her work at the morgue, realizes that the importance of the work outweighs her personal preference.
While at Columbia, Alexis is kidnapped with her friend Sara el-Masri while attending an event in town. While a frantic Castle and Beckett help the FBI search New York for them, Alexis is able to briefly escape and discover, to her astonishment, that they have been taken to Paris. She is able to phone Castle briefly before being recaptured. .
Because the el-Masris are a wealthy and political Egyptian family, it is assumed that Sara is the kidnappers' target. But it becomes clear that Alexis was the intended victim, as part of a plan to force her grandfather – Castle's long-lost father, Jackson Hunt – out of hiding. The plan works to some extent: Hunt does come out of hiding, but then teams up with Castle in Paris to save her from her captors, who are old enemies from Hunt's espionage past. 
While on a flight to London with Castle, Alexis becomes involved in helping her father investigate the murder of the air marshal, who is found dead on board, using some of her skills learned interning with Lanie. 
- Tweets by Molly C. Quinn 22:49 - 28 mai 12
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