Is Yaku Really 89 Years Old?

Yes. Yaku Yakuzen is canonically 89 years old. Her childlike appearance does not mean she is secretly the same age as Kusuri: Yaku is Kusuri Yakuzen's grandmother, has lived for nearly nine decades, and retains the memories, outlook, and life experience of an elderly woman. She looks roughly eight because she took an especially potent failed version of Kusuri's attempted immortality drug, which rejuvenated her body instead of producing true, flawless immortality.

Spoiler warning: This article covers Yaku's manga introduction, her past, her romance with Rentaro, and later manga developments involving the immortality drug that have not yet been adapted by the anime.

Yaku Is Really 89, Not an Eight-Year-Old Relative of Kusuri

The joke behind Yaku's introduction is deliberately built around a visual contradiction. Rentaro expects to meet Kusuri's grandmother and instead encounters someone who looks remarkably similar in age and stature to Kusuri's usual small form. The reveal is not that Kusuri has another young sister or cousin. It is that this is genuinely her grandmother.

Shueisha's official 100 Girlfriends character profile is explicit: Yaku is 89 years old, she is Kusuri's grandmother, and the failed immortality drug left her with the body of an eight-year-old. The same profile distinguishes her from Kusuri, who is 18 but normally appears eight because of her own failed drug.

That age shapes how Yaku is written. She remembers an adult life that long predates the series, including marriage, widowhood, medical work, and wartime service. Her youthful body is a drug effect; her identity, memory, and personal history were not reset with it.

Why Yaku Looks So Young

The Yakuzen family's most famous invention is Kusuri's attempted immortality drug. As with many of Kusuri's medicines, the label describes the intended result more neatly than the actual one.

Kusuri herself is 18, yet the failed drug normally keeps her in a body resembling her eight-year-old self. She can temporarily return to her 18-year-old form by taking a neutralizing drug, although the original medicine eventually reasserts itself.

Yaku's case is stronger. In her introductory manga material, she explains that she took a version of the immortality drug that was closer to completion. Instead of making her conventionally immortal, it rejuvenated her to approximately eight years old, and Kusuri's ordinary neutralizing medicine cannot simply restore her elderly appearance.

This is why Yaku and Kusuri can look like peers even though they belong to different generations. Their similar appearances come from the same pharmaceutical experiment, but their actual ages are dramatically different.

The motive behind Kusuri's research also makes Yaku's condition more than a random running gag. Kusuri developed the immortality drug in large part because she did not want her beloved grandmother to die. The failed result visibly represents that attachment: Yaku gains an enormously younger body, while the science still falls short of the clean, permanent immortality Kusuri originally wanted.

Yaku and Kusuri can both appear much younger because of the drug, but they are different generations of the Yakuzen family. Yaku is Kusuri's grandmother, not another child her age.

Yaku and Kusuri Are Not the Same Character

Because their designs intentionally echo one another, Yaku and Kusuri are easy to confuse in screenshots or spoiler discussions. Their roles are very different.

Kusuri Yakuzen is Rentaro's fifth girlfriend. She is an 18-year-old high-school senior and chemistry-club president whose failed immortality drug usually leaves her in an eight-year-old form. Her personality is energetic, impulsive, and inseparable from the bizarre medicines that regularly drive the manga's comedy.

Yaku Yakuzen is Kusuri's grandmother and later becomes Rentaro's sixteenth girlfriend. She is 89. Her youthful body comes from a stronger iteration of the same failed immortality experiment, but her behavior usually reflects her actual lifetime rather than her physical appearance.

Even their speech and tastes separate them. Yaku favors old-fashioned expressions and traditional habits, struggles with modern loanwords and technology, and often treats the increasingly enormous Rentaro Family with the patience of a grandmother observing excitable grandchildren. Kusuri is much more comfortable with the contemporary world even when she is behaving far less maturely.

So when a scene features an apparently young Yakuzen family member, appearance alone is not enough to identify her. Yaku's long-lived perspective is the real distinction.

Yaku Had a Full Adult Life Before Meeting Rentaro

Yaku's age is supported by more than a number on a profile page. The manga gives her a past that would make no sense if the "89" were merely a joke or fake identity.

She worked in medicine and served in a wartime environment when she was younger. Later material depicts her familiarity with treating severe injuries and developing medicines under circumstances far removed from Kusuri's school-laboratory antics. She also had a husband whom she loved and eventually lost.

That makes Yaku unusual within the Rentaro Family. When she meets Rentaro, she is not experiencing romance for the first time. She has already lived through a marriage and bereavement, raised a family line that eventually produced Kusuri, and reached old age before the immortality experiment radically altered her body.

Her wartime memories also explain why her competence sometimes seems disproportionate to her appearance. She is knowledgeable about medicine and medicinal plants, physically capable, and rarely overwhelmed by ordinary crises. When the manga wants to remind readers that the small figure in a kimono has lived for almost nine decades, it can do so without changing her design at all.

How Yaku Becomes Rentaro's Girlfriend

Yaku enters the story when Rentaro becomes involved with Kusuri's wider family. The manga formally introduces her as Kusuri's grandmother and then delivers the series' familiar soulmate "shock" between her and Rentaro.

The relationship does not erase Yaku's previous marriage. That is an important part of the way their romance is framed. Rentaro recognizes that Yaku had a husband before him, while Yaku does not ask him to become a replacement for the man she lost.

Instead, the appeal is that Rentaro can be loved as himself.

Yaku is also harder to fluster than many earlier girlfriends. Rentaro is accustomed to giving completely sincere declarations of affection with enough force to overwhelm almost anyone, but Yaku has decades of emotional experience behind her. She can tease him, observe his efforts with amusement, and make him work harder to understand what reaches her emotionally.

That creates a different rhythm from Kusuri's romance with him. Kusuri's relationship often runs on scientific chaos and exuberance; Yaku's tends to emphasize patience, experience, quiet teasing, and Rentaro's determination to take every girlfriend seriously regardless of how unusual the circumstances are.

Yaku is canonically Rentaro's sixteenth girlfriend, between Momiji and Kishika in the girlfriend sequence.

The intended illustration here should show that dynamic rather than making Yaku and Kusuri interchangeable: Rentaro and Yaku sharing a calm walk or tea break, with Yaku amused and self-assured while Rentaro earnestly listens to her.

Her Personality Makes the Age Reveal Work

Yaku's characterization constantly reminds the audience that physical appearance and chronological age have separated.

She is generally composed, affectionate, and difficult to shock. Rather than competing with the younger girlfriends for attention in exactly the same way, she often occupies a grandmotherly position within the group. Her warmth toward Kusuri is especially important because Kusuri's fear of losing her is one of the emotional foundations of the immortality-drug story.

At the same time, Yaku is not written as solemn simply because she is elderly. She enjoys teasing Rentaro, can participate fully in the family's absurd group activities, and has her own comic weaknesses.

One of the most recurring jokes is her difficulty with modern language and technology. English-derived terms, contemporary devices, and other pieces of modern life can leave her genuinely puzzled. The humor is based on generational distance rather than memory loss: she is mentally an elderly woman confronting a world that has changed around her.

Her traditional dress and old-fashioned speech reinforce the same point visually and verbally. The manga wants readers to remember that the apparent child on the page is speaking from a lifetime of experiences that the other characters do not have.

The "Immortality" Drug Is Not Perfect Immortality

Later manga canon adds an important qualification to Yaku's premise: the failed immortality drug should not be treated as proof that she is permanently immortal or eternally locked in one condition.

In the much later manga storyline around Chapters 235 and 236, the Yakuzen family's immortality-drug effect unexpectedly fails. For Yaku, that is far more dangerous than it is for younger relatives. Her body returns toward the condition associated with her true age, and her health deteriorates severely.

Kusuri desperately tries to create a solution. Rentaro helps her push the research forward through the series' characteristically absurd mixture of pharmaceutical science, supernatural comedy, and extreme devotion. They ultimately produce an improved immortality drug, and Yaku returns to her familiar rejuvenated state.

The arc confirms two things at once.

First, Yaku really is an elderly woman whose small body depends on the drug's effect. When that effect fails, the story does not reveal that she had secretly become an ordinary eight-year-old.

Second, calling the original medicine "immortality" should not be interpreted too literally. It was always a failed experiment, and later canon demonstrates that its protection can break down. Yaku's continued survival is maintained through the Yakuzen family's increasingly extraordinary medicine rather than established as an unconditional supernatural rule that she simply cannot die.

Has the Anime Introduced Yaku?

Yes. Yaku receives her formal anime introduction in season 3, episode 32, "Kusuri-senpai's Grandmother." Episode 31 sets up Rentaro's visit to the Yakuzen home, while the following episode introduces Yaku, confirms that she is 89, and brings her into the Rentaro Family.

Rie Kugimiya voices Yaku in the anime. The adaptation preserves the central manga reveal: the small girl Rentaro meets is Kusuri's grandmother, and her youthful body is the result of the failed immortality drug.

Anime-only viewers therefore receive the same answer as manga readers. Yaku's age is not an unadapted spoiler or a fan inference; it is now stated directly in the televised story.

There is no separate light-novel continuity that replaces the manga answer here. The franchise does have a spin-off novel, but Yaku's age, origin, and main relationship with Rentaro are established in the serialized manga. In that canon, there is no ambiguity about the central question: Yaku is chronologically 89, Kusuri is her granddaughter, and her eight-year-old appearance is the consequence of an imperfect immortality drug rather than her true age.