How do you get a free Prismana Aniimo?

The useful part is the word "choice". You are not rolled a species and told to like it, you pick one, and the one you pick is a rare Prismana. So the first guaranteed Prismana on your account is one you chose on purpose rather than one you fell into.

That is worth thinking about before you hit the rank, because of what a Prismana is. It is an exceptionally rare, radiant variant of a Nova-stage Aniimo of the same species, with a modified 3D model and an alternate color palette. How far the change goes is not settled: in some builds it reads as a purely visual variant, in others as an empowered form that differs by more than its looks. Either way you are locking in a species, so pick one you actually want to field.

This is not the only free one. A free rare Prismana also comes as a launch reward, separately from anything you earn by ranking up.

Getting Prismana Aniimo after the rank reward

Nurturing is the other route. How central it is has not settled down yet: it behaves in some builds as the main way Prismana Aniimo come into your roster, and in others as one option among several. Treat it as reliable but not necessarily the only road.

There is also the Prismana Orb, which turns an eligible Aniimo into its Prismana form. The catch is the condition attached. That form has to have been unlocked already, so an Orb converts something you have access to rather than handing you a new Prismana out of nowhere. That makes the free one from Junior Wayfarer and the launch reward more than just early gifts, since unlocks are what the Orb spends.

Not settled: Whether the launch reward Prismana and the Junior Wayfarer pick are two separate creatures or the same grant described twice is not clear from how the rewards are laid out.