Can you change your starter Aniimo?

The choice comes at you early. Picking between Lunara and Helion is the first major decision in the game, and it lands before you have much sense of how anything works. The good news is that it is not a power decision. The two starters share mirrored stats and fill the same combat role. Neither one is the stronger pick, and neither one leaves you short in a fight later.

What actually separates them is play style and appearance. So choose on how you want to be playing, not on which one looks like the better investment, because there is no better investment to find. Appearance is the weaker reason of the two anyway. An Aniimo's look and form can shift with the region you are in, the environment, the weather and the time of day, so the version on the selection screen is not the only version you will see.

Your starter does not lock how you get around

The permanence matters less than it first sounds, because your starter is not the only creature you act through. Twining into different Aniimo grants specific mobility skills and changes how you navigate the world. Traversal comes from what you twine into, not from the companion you picked at the start, so committing to Lunara or Helion is not committing to one way of moving through the map.

Not settled: Whether the other starter can be obtained later by any route other than a fresh start is not settled.