The flashback portion of the short film shows a younger Rin interacting with her father. It's clear he loves his only child through his actions of playing with her in a sandbox, brushing her hair for her, and giving her a giant teddy bear to remember him by. He goes as far as building her a spaceship so at least she'd be able to escape the oncoming apocalypse.
Learning that her father's dead and that she's (most likely) the last human being alive would crush the spirits of anyone else, but instead Rin manages to put a positive spin on her situation: yes, she's technically all on her own, but so long as she has the memories of the time she spent with her adoring father, she'll never be truly lonely. She also thanks him for everything he did.
