Regarding the ex-girlfriend in Episode 3: She said, "In the three years I was with him, I felt like his heart was always somewhere far away." How does the existence of this ex-girlfriend, who only appears indirectly through text messages, contribute to shaping Takaki Tono's adult character?

This is an exceptionally insightful question that highlights a character easily overlooked yet crucial to understanding Takaki's adult persona. This ex-girlfriend, who appears only indirectly through a single text message, functions like the most pivotal witness in a courtroom. Her "testimony," in the most concise and brutal manner, delivers an irrefutable verdict on Takaki's "stagnation" and "emptiness."

Her significance manifests primarily on three levels:

1. She is the Judge from the "Real World"

Throughout the story, Takaki's inner world is completely open to the audience; we understand why he suffers and why he is lost. However, this "knowledge" easily draws us into sympathy for him, potentially obscuring the essence of his actions. This ex-girlfriend represents the innocent real world that he has harmed.

Provides an External Perspective: Her existence shatters Takaki's "victim" image. For the first time, we see Takaki's true state from the perspective of an "other" who had a deep relationship with him. He is no longer just a lovesick boy, but an adult who has failed in real-world intimacy.
Objectivity of the Accusation: The text message contains no harsh accusations or insults, only a calm yet utterly weary statement: "It felt like his heart was always somewhere far away." This calmness is more powerful than any vehement reproach. It signifies this wasn't a momentary quarrel, but a feeling accumulated over three long years. It transforms Takaki's "problem" from a romantic "literary device" into a heartbreaking "real-life tragedy."

2. She Embodies Takaki's "Emotional Paralysis"

Without this ex-girlfriend, the audience's understanding of adult Takaki might remain vague, limited to notions like "he is numb at work" or "he is lonely." Her appearance precisely reveals the concrete manifestations of this numbness and loneliness within his emotional life.

From "Devotion" to "Selfishness": This text message jolts the audience awake: Takaki's "devotion" to his memories of Akari is, in reality, extreme "selfishness." He enjoys his girlfriend's companionship while withholding his genuine emotions. He seals off his inner self, forcing someone who loves him to endure the pain of "loving a ghost" for three long years.
Tearing Off the Mask of "Gentleness": We always knew Takaki was gentle. But this text message reveals that when gentleness loses its core of sincerity, it becomes the cruelest weapon. His gentleness is an "emotional barrier" against those close to him, a form of cold violence that says, "I won't hurt you, but don't come near me."
The Weight of "Three Years": The detail "dated for three years" is crucial. It shows Takaki did try to "move on"; he made an effort. The result? Three years proving he couldn't escape the prison of his memories, while draining another person's youth and emotions. This profoundly deepens the tragedy and complexity of his character.

3. She is One of the Catalysts for Takaki's Ultimate "Growth"

Though a breakup message, it indirectly becomes a driving force pushing Takaki towards eventual acceptance.

Sounding the Alarm: This failed relationship and its "verdict"-like text message inevitably trigger Takaki's self-reflection. He realizes his problem is no longer simple "longing," but something severely impacting his real life and causing tangible harm to others.
Cutting Off Retreat: Breaking up with his girlfriend leaves him utterly alone. He no longer has a "safe harbor" in reality to hide in, forcing him to confront the ruins within his own heart. This provides the necessary external conditions for him to finally make peace with his past self at the story's end.

Conclusion: A Perfect "Character Sketch" Tool

In summary, this ex-girlfriend, appearing only through a single text message, is a stroke of narrative genius by Makoto Shinkai—exceptionally efficient and masterful.

She is like a precise surgeon, making a single incision to slice through the seemingly calm surface of adult Takaki, revealing the complete necrosis of his inner emotional functions.

Her existence transforms Takaki's image from a two-dimensional "lovesick soul" into a three-dimensional, conflicted, flawed "ordinary person" worthy of sympathy but also deserving critique. She deepens the realism of 5 Centimeters Per Second and makes Takaki's relieved smile at the end feel hard-won, carrying the true weight of growth.