Rocky is a rock-shaped alien. I don’t think it’s a spoiler to say that; the trailers were generous in showing him.
Animated characters are similar to Rocky’s design: a voice actor provides one aspect of his personality; the illustrator (or in this case, the puppeteer) provides another layer. They can have an Enneagram, and so can Rocky. However, because he has no human-style face, the voice carries more of the load.
James Ortiz gives Rocky a very energetic voice; he loves problem-solving and is curious, asking many questions. He also has a dry sense of humor. He’s loyal to his species, his mate, and his new friend Grace. Rocky has a generous heart.
He could be any of the traits: Head, Heart, or Body Type. Because he’s the only Eridian we get to know, we can’t tell if he’s more physical than others, more sympathetic, or more cerebral. He’s our only example. With so many options, it’s tempting to call him a Null.
That wouldn’t be fair, though; he clearly has a personality. We’ll need to judge him by human standards.
His wit and exuberant physicality suggest a One. His adventurousness suggests a Seven. His sorrow at being alone in space suggests —
A Two. His little xenon sculptures are his Two Collection. The last half of the movie makes clear that Rocky is a Heart Type. I’ll say no more than that.