In Episode One, remember how Heatwave said he wanted to leave? Well, the real reason behind it was way different than the one he gave.
He actually didn't want to leave, he thought maybe the team could really work with the humans, but Heatwave didn't want to get attached. He didn't want to accidentally act up and then forget everything during his time working with them, and he didn't want to risk losing his memories of it if he stayed due to how his spark is.
So, he tried to get all of them to leave, but obviously that never ended up happening since Cody did something.
As the team stayed on Griffin Rock, Heatwave actually did some Journaling (after he found out what it was) of each major emergency (literally each episode we see), so just in case he forgot due to his sparkbreak situation, he could always look at it to at least get an idea of what went on.
In there, he also has descriptions of everyone. Those descriptions contain things that Heatwave would never speak about the others, the kinder things he is reminded of in each one of them.