“I am sad also. But we not be sad for long. You are scientist. I am engineer. Together we solve.”
Grace and Rocky's friendship, full stop.
This is especially heartwarming when you realize that Grace was rather lonely on Earth and had trouble forming deep, meaningful relationships due to fear of failure and disappointment.
Given their professions and demeanor, Grace and Rocky are both curious and science-orientated people. Hence, when they ask about each others' planets, they aren't only getting information for their scientific communities but also massively nerding out together.
Rocky and Grace each exposing themselves to near-fatal injuries to save one another during a massive ship failure after getting a sample from Adrien.
Rocky's insistence to keep Grace on a regular sleep schedule. Even if that is motivated by dangerous mistakes Grace did earlier when sleep-deprivated, it is still caring for a friend.
After a huge breakthrough, Grace and Rocky celebrate best they can; Grace brings out vodka, while Rocky wears his formal clothes. And Grace compliments how Rocky looks, and Rocky thanks him.
After the two friends left on their ways to bring taumoeba to their respective solar systems, Grace looks at the taillights of Rocky's ship as long as his sensors spot them. Also tear-jerking.
Grace really nails his Character Development when he decides to turn around his ship and go save not only his friend but also his friend's species, throwing away his opportunity to go home.
Epilogue: While it is sad that Grace didn't get to return to Earth, he is joined with his best friend Rocky. Grace also gets to return to the job he loves, teaching (Eridian) children. In the end, Grace receives confirmation that Earth was saved as well, meaning that he saved two planets from catastrophe.
Despite not being a father, Grace is a Friend to All Children and found his calling as a well-liked middle school science teacher. It is endearing how he has put much work to be the cool science teacher, including a correctly scaled solar system in his classroom and giving fun prizes to kids who excelled in his class. He knows very well that his students can be unruly and cheeky, but he loves them and wants their best.
He also instictively refers his students as "his kids", which makes other people mistakenly think that he is talking about his biological/adopted children. In fact, he confused even his own amnesiac self! When he remembers that he actually isn't a father, he is momentarily horrified, until he remembers his profession.
His love towards his students is also what motivated him to continue with the astrophage research. He was ready to leave it to others, until it hit him how badly they were going to suffer as a result of the disaster. This realization causes him to walk out of the school, drive directly back to Stratt, and demand to be allowed to work on the program.
He even gets to teach (Eridian) children when he settles down to another planet!
