Mayu and Takuma have known each other since they were young children. She's the daughter of a talented doctor, and he, one of his patients, suffering of a heart disease. Despite always being hospitalized, Takuma always had Mayu as a gateway for forgetting about his tough life. Eventually, they end up falling in love, and they promise to marry each other once Takuma gets better and they become adults.
The only downside is... Takuma's sickness may not let him live to become twenty years old. Mayu and Takuma will have to learn to live with a limited relationship, where everything, even time, seems agaisnt them, even if there's the faintest hope that at the end they may be together forever.
Thus begins Boku no Hatsukoi wo Kimi ni Sasagu (lit. I Give You my First Love), a shoujo manga by Kotomi Aoki that ran in the Sho-Comi for 12 volumes. It was originally conceived as a spin-off for the author's earlier series Boku wa Imouto ni Koi wo Suru, where Mayu and Takuya where supporting characters, but it quickly becomes a stand-alone work outside from cameos of Boku Imo's main characters in a few chapters. The story distantiates itself from other drama and romance stories by following Mayu and Takuma's relationship from when they meet as children until they become adults, passing through their school years. The popularity of the manga was huge on its day, becoming one of the best-selling series in Japan and earning the 57th Shogakukan Manga Award in the shoujo category. It also inspired a live-action movie in 2007.
- Artistic License – Law and Artistic License – Medicine: There's no way that after the car accident the dead little girl's tutors did not sue Kou for his responsibility. As part of the procedure, he should've had a medical test to ensure his well-being and sanity where the brain hemorrhage would've been discovered.
- Beta Couple: Yuiko and Ritsu.
- Billy Needs an Organ: Eventually, the condition for Takuma to get better is to get a heart transplant. Kou gets appointed as the donor, but both Takuma and Kou's family reject it.
- The Cameo: Iku and Yori from Boku wa Imouto ni Koi wo Suru appear in a few chapters after Takuma is appointed as dorm leader.
- The Casanova: Kou.
- Childhood Marriage Promise: Mayu and Takuma did this as children. Mayu made him promise to marry her as a way to keep him from dying.
- Convenient Coma: Kou goes into brain death state due to the sequels of the car accident, because he didn't get his injuries examinated. This allows Takuma to get a donor for a heart, but both him and Kou's family reject the transplant, considering Kou is still alive.
- Covers Always Lie: The volume covers show Mayu and Takuma as a average shoujo couple, doing normal things like going on dates or kissing on romantic spots. Most of these scenes don't occur in story, since Takuma's sickness prevents them of doing so.
- Definitely Just a Cold: Kou suffers numerous headaches after the car accident, but he brushes them off saying he's just tired. They're symptoms of brain hemorrhage, that end up making him faint and enter brain death state.
- Disappeared Dad: Kou and Ritsu's dad died from a similar heart sickness.
- Four-Leaf Clover: Mayu tries to find one when she first learns about Takuma's disease. As she fails, she and Takuma buy an amulet with a clover on it when their elementary class go on a school trip.
- Delinquent Hair: Takuma dyes his hair blonde before entering middle school as a way to give other people something to talk that isn't his disease.
- Ill Girl: Teru.
- Japanese Delinquents: Takuma isn't one per se, but has the looks: blond hair and piercings. This was an studied look to avoid being ignored in school.
- I Want My Beloved to Be Happy: For most of their middle and high school years, Takuma avoided and pretended to hate Mayu so she could forget him and thus avoid being broken-hearted after he died from his disease.
- It's implied Kou deliberately didn't get his injuries examinated so he could die and Takeru could get his heart transplanted, making Mayu happy.
- Ladykiller in Love: Eventually, Kou falls in love with Mayu for real, but he decides to move on from her. However, it's very much implied he forced his condition to worsen so he could die and give his heart to Takuma.
- Megane: Teru and Ritsu when they're high schoolers.
- No Ending: The last chapter is about Takuma entering into surgery while Mayu reads his last will: we get to see many scenes of Mayu and Takuma being together happily as a healthy young couple, even effectuating a traditional Japanese wedding and visiting the clover field from the first chapter with their children... but we are never explained if this was just all an Imagine Spot Takuma thought while writing the letter (meaning he died during the surgery or shortly after) or Mayu and Takuma's actual future (meaning the surgery succeeded)
- Rapunzel Hair: Mayu, by the time they're in high school, has hair that goes past her waist.
- Soap Opera Disease: Takuma suffers from a heart condition that doesn't allow him to do any physical effort nor getting him too excited, as it causes him to faint. Other characters, like his childhood friend Teru and Kou and Ritsu's father, suffer it too.
- Their First Time: Mayu and Takuma lose their virginity in their night alone during their high school trip. Igarashi, Kou's classmate, loses it to him after he decides to move on from his crush on Mayu.
- Your Days Are Numbered: Takuma's expected not to reach adulthood. He may die before he becomes twenty.