In <i>Being and Time</i>(<i>Sein und Zeit</i>), Martin Heidegger points out that the personality of a person who is aware of their mortality is individualistic, and they seem not to have relationships with others. By contrast, in <i>The Human Condition</i>(<i>Vita activa oder Vom tätigen Leben</i>), Hanna Arendt emphasizes the relations between others as "action"(Handeln).<br> In this article, I first of all endeavor to compare the "being-with"(<i>Mitsein</i>)in <i>Being and Time</i> with the "action" in <i>The Human Condition</i>, and from there attempt to reinterpret the <i>Dasein</i> as a performer of the action. This argument indicates that we are "with" others precisely in actions involved in unpredictable possibilities.<br> Secondly, I focus on authenticity, which is disclosed when the <i>Dasein</i> is anticipating death and the existential structure of our "who" is revealed. It would appear that "individualization" and "no relation" negate being with others in the world, but this existential metamorphosis into authenticity is our transformation into <i>Dasein</i> as a performer of action in the world with others through our becoming aware that we are each a unique "who" who cannot be defined in the world.<br> Death as the most extreme possibility is neither a predictable possibility nor an unpredictable possibility on the line that extends from the present into the future, but it shows that we ourselves are beings with potential in the sense of making these possibilities.<br> By clearly showing the relationship between authenticity and the performer of the "action", I endeavor to point out that the thoughts of Heidegger and Arendt are not opposing, but rather complement each other. Finally, this article shows that it is precisely because plural "whos" who are the most unique "beings with potential" when anticipating death face each other that actions have the characteristics of newness and initiative as if they were a "second birth."