-Tsukki here is still as condescending as he can be to people, but he’s pretty gentle with animals.
-An injured seal near the shore catches Tsukishima’s attention, so he brings it to land to tend to its wounds. He makes the seal stay there until the latter’s wounds heal.
-The next day, though, instead of an newly-tended seal, he finds a boy who looks around his age, and with eyes with color like the deep part of the ocean, holding the tattered skin of the seal Tsukishima tended to the day before.
-The boy recognizes Tsukishima as the one who saved him, so he goes with the other to temporarily take shelter in, out of gratitude.
-“Don’t worry, I won’t steal your seal skin. It will take days to be repaired though so just stay here for a while.” A while meaning a week or something.
-The boy doesn’t actually speak here at first, but he does make noises. And he learns how to speak some words from Tsukishima.
-“Ball”. He quickly finds a ball in Tsukishima’s house and had fun playing around with it. Tsukki would find it amusing and wonder if this had anything to do with the boy being a seal.
-“Food”. When Tsukishima comes home from a day spent fishing, the seal boy comes to him and stares at the container he brought home with fish inside. It’s enough of a sign that he’s hungry.
-“Kei”. When the boy is eager to learn more than just ‘ball’ and 'food’, he’s curious about Tsukishima’s name. He also pointed to himself and uttered “To-bi-o” after learning how to say “Kei”.
-Before Tsukishima knew it, Tobio’s the first person (technically a seal, whatever) he isn’t arrogant to. In fact he enjoyed the other’s curious company, to the point of being attached.
-At that same point, Tobio yearns for his home by staring longingly at the ocean. “Sea” and “Home” are added to his vocabulary, thanks to Kei who is now anxious if they will ever meet again if the other’s seal skin is already healed.
-A few days later, it did finally heal. In fact, Tsukishima is more surprised that Tobio did not just disappear, but instead actually waited for him to wake up to bid farewell.
-Tobio touching the other’s chest (above his heart) and uttering the words “Kei” and “Home” were enough to at least tell Tsukishima that they will see each other again. And it’s not a permanent goodbye, even if he sees Tobio off to the ocean, carrying his now-healed seal skin.