There's not much shown about Hiratsuka's past but during it has been hinted throughout the series that in her youth, Hiratsuka was as much of a loner and antisocial as Hachiman was and she's been shown to lament it.
It has also been hinted she has been hurt deeply by her friends in the past and that she doesn't want Hachiman to have the same fate happen to him.
During her university days she used to be in charge of starting the fire for bbq/campfires when out with friends. While she's busy tarting the fire, the couples in her group started to do their lovey-dovey stuff that would irritate her showing she was already upset about being single. Shizuka states that it was her own "miscalculations" that led her to this single status.
In Volume 2 she hints at a past lover that just used their relationship for a place to stay and when he left he took some of her money and furniture.
In Volume 10 she mentions she didn't actually study to be a teacher (dialogue and her lab coat hint she might have wanted to be a pharmacist).
When Hachiman clashes ideals with Yukino, Shizuka proposes a contest. They will use their ideals to help people and whomever helps the most people will win, with the winner being able to order the loser around.
Shizuka approves when Yui wishes to join the club.
She later reveals to Hachiman that her real goal is to take antisocial students and observe them as they are re-socialized. Hachiman compares the service club to a Sanatorium. She mentions she will keep a close eye on the Service Club and its members over the school year.
Shizuka enters the Service Club one day and updates Hachiman and Yukino on their contest. She considers the score 2-2, a tie.
Shizuka is disciplining Hachiman for his absurd Workplace Tour Application when Yui and Yukino come looking for him. Yui asks for his phone number so she can just text him next time. Hachiman mentions he's texted other girls before, but now Hachiman realizes he was only getting replies out of pity. Shizuka feeling genuinely sorry (and as the school counselor) adds her number to Hachiman's phone as well, offering to talk anytime.
During the workplace tours Shizuka tells Hachiman that she has to revise the rules of his and Yukino's contest. She has to think of a way to accommodate Yui.
Shizuka heads to the Service Club to tell them about the new rules for the "battle-royale". She mentions it's now a 3-way brawl, and they are allowed to cooperate. This will let them oppose each-other, yet learn to work in tandem at times. But before the new rules can work, they need to recruit another person. Shizuka threatens Hachiman and Yukino to find another member by Monday.
Yui invites Shizuka to her karaoke birthday party. Shizuka declines because she is busy.
Later on the Service Club runs into Shizuka at karaoke anyway. Yui guesses she went to a singles meet that didn't go well, so she ended up going to karaoke solo. However, its hinted she was there the entire time in the room next door because of the occasional disturbances the Service Club heard.
Shizuka "volunteers" the Service Club and Hayato's clique to help at the Chiba Village summer camp. She has trouble getting in contact with Hachiman and has to bend the rules to contact Komachi. Komachi agrees to help and tricks Hachiman into going to the camp.
At the Summer Camp, Shizuka is shown to be in charge of some of the events and is planning the end of camp "festival".
She allows the Sobu students to try and help Rumi. She is aware of Hachiman's plan for the test of courage and reminds him that even one small misstep may lead to trouble. Regardless she lets the plan continue.
One day during Summer Break, Shizuka attended her cousin's wedding. While there her parents and relatives bothered her about her own romantic endeavors, and if she is getting married any time soon. She noticed Hachiman passing by and used him an excuse to leave as she was the teacher for the "problematic student" (Hachiman).
She accompanies Hachiman to get ramen. While out they find themselves talking about Haruno, Yukino, and the upcoming firework festival. She also praised Hachiman for doing club activity over summer vacation (giving Taishi advice), and reveals that she gets information from his sister Komachi.
Afterwards, she promises to take Hachiman to every ramen shop in Chiba once he graduates, since its weird for a teacher to be hanging out with a student.
When Hachiman is absent from class, Shizuka picks him to be their class' representative for the Culture Festival.
Shizuka along with the gym teacher Atsugi are the teachers overseeing the Culture Festival committee. Its hinted that Shizuka sends Sagami to the Service Club for help on the Culture Festival.
She shows concern for Yukino, who hasn't turned in her 3rd year course choice yet. She agrees to wait until after the Culture Festival for Yukino's career survey.
Later, when buying time for Hachiman to find Sagami, she plays the bass in the 2nd volunteer band group.
After the Culture Festival, Shizuka praised Hachiman for his effort and results, but not his actions. She is disappointed that he continuously makes himself the scapegoat or victim. She warns him that if he keeps doing it he may ruin his future.
After the Culture Festival, Shizuka leaves a laptop and a note for the Service Club. She informs them they will now consult with people through email, "The Chiba Prefecture Problem E-Consultation".
When the Service Club agrees to help Meguri with the Athletic Festival, Shizuka reveals herself as the teacher consultant and the one that suggested them to Meguri.
She initially was uneasy that Sagami and not Yukino, would be the committee chairperson, but she trusts their judgement in choosing a capable leader.
While choosing ideas for a special event Shizuka keeps dismissing everyone's suggestions. She wants to play it safe and avoid any possible punishment for heerself. However, she agrees to let the Service Club "outsource" the event ideas to Zaimokuza and Ebina.
Throughout the planning committee Shizuka provides advice and a guiding hand to Meguri, Sagami, and the Service Club for the many problems that arise. Her constant presence contributed to the Athletic Festivals success.
During the Athletic Festival events, She was in charge of getting everyone's attention and organizing start times. She started each event by blowing a whistle or large conch shell.
Volume 6.5 Drama CD
She advises Hachiman and Yukino to mess up as much as they want as she is watching over both of them.
After dinner she lets Yukino and Hachiman walk back to the hotel and goes to buy sake for herself.
Shizuka is tasked with writing a column for a Chiba magazine about love and marriage. Since she is clueless about relationships she turns to the Service Club for help. They in turn rely on Komachi who hosts a "bridal/wife contest". During this event Shizuka demonstrates her beauty and cooking skills as well as her unusual views on marriage. Komachi was going to pronounce herself the winner of the contest but Shizuka intimidated Komachi into awarding her the prize.
In the latter half of the LN, it is mentioned that Shizuka sent the Judo club members to the Service Club with their request.
Volume 7.5 Drama CD is included in volume 7.5 as a bonus track.
Shizuka asks the Service Club to help Isshiki lose the student council presidential election. She says she will punish the pranksters that nominated Isshiki. She also takes note of how strained things are between the Service Club members.
Later, she declares the "battle royale" to be a draw still, and finally informing Yui that she is part of the contest as well. She also tells Hachiman that he is trying to do too much, and if he continues doing things the way he is, in the end he wont be able to save someone that he really wants to.
Shizuka accepts that each of the Service Club members have a different approach to Isshiki's request. She is the one that Yukino reports Isshiki's "presidential win" to.
Shizuka encourages Iroha to work with Kaihin Sougo to create a Christmas event, which she becomes the supervisor for. She is the one ultimately approving their budget and any ideas they might have. Hachiman believes she did this knowing Iroha would seek the Service Club's help with organizing the event.
Shizuka checks in on the collaborators when Hachiman is there, she is surprised that he is alone and the other Service Club members are absent.
With the deadline one week away and being in charge of the event, she goes to check on the planing committee only to find they have already left. On her way home she finds Hachiman walking, who is depressed about recent events and offers him a ride home. She takes a detour to a bridge where she asks Hachiman about his situation. Hachiman starts explaining the mentality of planing committee impressing Shizuka, but Shizuka points out the flaws in his logic about solving things and gives him a hint to solve his problem.
Later, Shizuka offers the Service Club and Isshiki four tickets to Destinyland, in order to help them understand Christmas celebrations better. Shizuka praised Hachiman for expanding his social circle and is glad she could witness it personally as she feels that she could not watch over all of the club members for ever. She also felt happy that being in the Service Club brought positive changes to group members and hopes that they will even gets better in the future.
After classes one day Shizuka goes to the Service Club with Hachiman. On the way she explains she was going to get the Service Club to help with an extracurricular study program. She also praises his grades and mentions she cant help him find out Hayato's career path. They also talk about Hachimans career path. He wants to choose Liberal Arts, but has no idea what to do after high school. Shizuka reminds him that you never know how things will turn out, mentioning she didn't study to be a teacher. However, she advises him on giving up his dream of becoming a househusband. And then mentions she thinks he'd be a good teacher, which he refutes.
For the student marathon Shizuka is the one with the starting gun. Hahciman notices and wonders if she likes to do things to stand out, or if she just wanted to shoot the gun.
Shizuka approves the Service Clubs request to have their heater fixed.
When Isshiki wants to start a student newsletter she drags the Service Club with her to ask Shizuka for permission. Shizuka approves their request to print and post newsletters around the school grounds. However, she has to review everything before they print it.
Shizuka is constantly reminding Isshiki to finish and let her review the graduation speech as its only a month away. She also reminds Isshiki that next year the Service Club members will be in third year and wont be able to help her all the time. Isshiki insists she is the on helping this time, and explains the Pre Valentine's Day event. After hearing this Shizuka softens her stance and allows Isshiki to take a break from graduation planning in order to help the Service Club.
On the day of the event Shizuka makes an appearance. She read Isshiki's reports and decided to check things out. She reminds the Service Club of her own rule about chocolates being banned on campus, as well as her having a date on Valentine's Day, but then adds a "just kidding," before wandering off.
She also makes Totsuka and Zaimokuza bring things to the event from school. They turn out to be very high-end chocolates (Godiva, Pierre-herme, Charbonnel, Sadoharu Aoki) that she bought to hand out to everyone. Everyone is surprised that Shizuka likes sweets and Haruno suggests that she and Shizuka could go out for chocolate, drinks, and girl chat sometime. Shizuka says she cannot do it that day.
Later Shizuka praises Hachiman and how much he's changed, although he refuses to admit it. She encourages him to keep making progress for the better. She then says the event has been wonderful and fun to observe before heading back to school to finish some work.