I'm assuming from the mention of The Drowned Road that you're talking about 1e Scion, since it doesn't exist in 2e anymore.

1. Ymir in 1e is mentioned as a Titan but nowhere do we have an answer as to which Greater Titan it actually belongs to.

2. I don't think Prithvi is ever mentioned in 1e, but going by that game's internal cosmology, she likely would be in Terra should you choose to use her.

In 2e Ymir is mentioned as a Primordial, and Prithvi likely is one too.