| 柏崎 星奈|
|Date of Birth||May 19th|
|Job||High School Student|
|Class|| 2-3 (Formerly)|
|School||St. Chronica's Academy|
|Japanese Voice||Kanae Ito (Japanese)|
|English Voice||Jad Saxton (English)|
|Novel Debut||Chapter 2|
|Manga Debut||Chapter 2|
|Anime Debut||Episode 1|
Sena Kashiwazaki (柏崎 星奈, Kashiwazaki Sena) is the female tritagonist of the Boku wa Tomodachi ga Sukunai series. She is the first recruited member of the Neighbor's Club and the daughter of Tenma Kashiwazaki, headmaster of St. Chronica's Academy. Her wealth, beauty, and outstanding academic and athletic performance have made her popular among male students, and likewise, have made her an outcast among girls. Despite being constantly surrounded by boys (who, according to her, are "doormats"), all she wants are true friends, which drove her to join the Neighbor's Club.
Before joining the Neighbor's Club, Sena doesn't have any female friends, because she is envied by other girls, due to her good looks and popularity with the opposite sex.
Sena has stated that she inherited most of her looks from her mother. Sena is a beautiful young woman with a pair of blue eyes. She has light blonde hair that reaches her mid-back, while it only reaches her upper chest in the front. Her bangs hang just above her eyes. Sena has a curvaceous body, is very well-endowed (Yozora insults Sena because of this, nicknaming her "Meat"), and tends to flaunt herself. Sena almost always wears her blue butterfly hair clip on her head and is usually depicted in her school uniform. When in public, Sena wears flashy clothes that fit her well. In the 6th volume, it is revealed that her butler, Stella, buys her clothes, picks out her outfits, and even does her hair and make-up every day.
On the outside, Sena appears to be an ordinary high school girl but in truth, Sena is arrogant and prideful by nature and is well aware of this, once even stating that she should do so since she's that good. Sena considers herself to be the school queen or goddess and so often treats her fellow male students as servants, or "doormats" as Sena herself has said (not that they mind). Although because of her personality and beauty, she has no female friends.
Sena also believes that there are two types of girls; the ones who are willing to be her friends and the others who are not worthy of her. Sena also does not treat the boys in her class with much respect due to them constantly ogling her, and slavishly doing whatever she tells them to.
However, whenever Sena's ego is shaken by someone (usually Yozora), she bursts into tears and flings childish insults before running away. Sena was surprised to see that Kodaka does not act like the boys in her class (wanting to be stepped on) and hence, views Kodaka in a different light. She first saw him in this different light when he rescued her from some guys who were harassing her at a public pool. Since then, her feelings have increased to the point where she nonchalantly admits that she wouldn't mind marrying him and that she is in love with him.
A perfectionist by default, Sena always tries her best at everything, and is willing to go to great lengths to do something perfectly (once playing a video game without sleeping until she had finished it). Sena gets so involved in her games that she becomes very angry if they are insulted in any way.
Sena is very gullible and often fails to see through the tricks Rika and Yozora play on her, even when the resulting humiliation is plain to see, (an example being the "heap" hairstyle). She only realizes her mistake when her father or some other person points it out to her.
Kodaka has noted her obsessive personality; that she becomes extremely engrossed in things she is interested in, sometimes to beyond reasonable levels (as seen with her grudge against Yozora), while completely ignoring things she is not interested in. Since she possesses both the confidence and the means to pursue whatever she may desire, he calls her "a real live princess".
Despite her princess-like arrogance, it's been shown that Sena genuinely cares for the other members of the club. For example, Sena offered to purchase a theme park ticket for Maria so that she would not be left out of the group, which showed a degree of care unexpected from an arrogant princess who thinks nothing of others. Later, while she exacted her revenge on Yozora for plagiarizing, Sena made a point not to go too far and offered Yozora a chance to redeem herself by giving her a part in the movie.
Sena also has an overly creepy obsession regarding Kodaka's younger sister, Kobato, going so far as asking to take a bath with her. Sena will go to any lengths to make Kobato happy, much to Kobato's discomfort.
- Meat (By Yozora)
- Little Miss Perfect (By Yozora)
- Sena-sama (by her male classmates)
- Niku (By Yozora) 
- Fog Lights (By Yozora)
- Sena the Gross (By Yozora)
- Twins (By Yozora) 
- Boobella 
- Sow (By Yozora)
- Breastables (By Yozora)
- Sand Bags (By Yozora) 
- Tits (By Yozora)
- Titzilla (By Yozora)
- Lumpy (By Yozora)
- Jiggles (By Yozora)
Sena is highly intelligent, being able to grasp every single lesson in great detail and hence, she always gets perfect scores for every single test and becoming the valedictorian of her year. Sena is even able to remember things from the past easily (as shown when she recalls the first time Yozora calls her by her own name) without much trouble. However, due to her pride, Sena sometimes claims to be very knowledgeable in areas she knows nothing or very little about and this often backfires on her.
However, Sena is also very naive and gullible often falling for Yozora's tricks and lies (although seen through easily by Kodaka and Rika).
It should be also noted that she shows great results not only in academic subjects but also in athletic ones. Sena is a fast learner, despite being unable to swim when the story starts, she is able to easily swim after a few lessons from Kodaka and even being almost as good as him on her first try.
Sena is very bad at domestic work, unable to do her hair and makeup since her family steward, Stella, does all of this for her.
- Sena's surname, Kashiwazaki (柏崎) means "Oak" and "Small peninsula". Together, her surname means "Oak trees in a small peninsula".
- Sena's given name (星奈) means "Starlet".
- Sena's supposed bisexuality is one of the most visible running gags in the series, especially when it comes to her rivalry with Yozora, which quickly develops into a natural obsession. For example, she attaches Yozora's photos to the ceiling (explaining it by preparation for "revenge"), sniffing her wig and in one of the spin-offs, even trying to kiss Yozora in the heat of a quarrel, vaguely trying to prove to her the "importance of romantic stuff".
- It is also hinted at several times in the series that Sena has a foot fetish.
- She makes a cameo with Yozora and Maria in chapter 40 of the manga Kämpfer. She is being teased by Yozora and Maria in the locker room.
- "That Idiot, Yozora!!" (her usual teasing towards Yozora)
- "I said this before, but the promise our dads made, or the fact that we met when we were little doesn't matter, But it's only the promise itself that doesn't matter. When we talked about it before it was like we just canceled the whole marriage thing entirely, but promise aside, I really do feel like I want to marry you. I mean, after all, I love you, Kodaka." (Sena unwittingly confessing her love to Kodaka) 
- "Quarrelling is like the normal operations of a machine. That's us, and a normal daily sight for the Neighbor's Club. Our relationship won't break over from a serious fight, I really think so." (Sena generalizing the relationship members of the Neighbor's Club share amongst themselves)