What does the starter Aniimo's passive do?

The 30% applies to any hit where you lack the elemental edge, including disadvantaged ones. It switches off only in the matchups where your element already beats the target's. Your starter keeps hitting hard against the enemies it has no answer for, so you don't have to swap out every time the elements turn against you.

Helion and Lunara are both Light Aniimo (Light is also called Holy), so the bonus triggers against exactly the same enemies for both. They also share identical base stats (ATK 116, 514 total) and the same DPS role. The passive gives you no reason to pick one over the other, and Lunara's safer ranged play does not cost her any damage. If you regret your pick, every starter option is a permanent Prismana Aniimo and can be obtained later through Prismana Flow.

The traits that do differ between the starters

On top of the shared bonus, each starter has its own signature trait, and these are what change how you play.

Helion's Solar Grace grants the Solar Halo buff on every fourth basic attack, and Solar Halo empowers his next active skill. In practice you count your basic attacks and fire a skill once the buff is up. His ultimate transformation also amplifies the damage of his other abilities, which fits a close-range burst and combo style.

Lunara's Lunar Power runs the other way around. Casting a skill or her Ultimate gives her four stacks of Silver Moon Mark. Each basic attack then consumes one stack, deals 100% additional damage and restores some HP. So you cast first and follow up with four basics. The healing on those hits is why she can stay in long fights from range without needing to retreat.

Not settled: How much HP each Silver Moon Mark hit restores is not settled. It is given as both 0.01% and 0.5% of HP, so treat Lunara's self-healing as real but of uncertain size.