It would be Low Hypoverse because it's below 0-D
As for Infinitesimal, I'm not entirely sure what you're referring to because it isn't a tier. Depending on the context, infinitesimal could be a lot of things. Infinitesimal isn't a fixed quantity like Infinity; it can mean a quantity that is less than any finite quantity but is still greater than zero, it can mean a quantity that is approaching zero but never reaching it (such as when computing derivatives or integrals in calculus), it can mean a quantity that is significantly smaller than another quantity (for example, 1 would be infinitesimal in comparison to infinity and a Planck length would be infinitesimal compared to a meter). Depending on how you take this, it could go from 11-C (points can be seen as having infinitesimal size in some contexts) to the absolute bottom of 11-B (due to being slightly larger than a 0-D point, which is just zero in other contexts) to the absolute bottom of 10-C (Infinitesimal energy could still mean finite energy in a 3-D sense).
Basically, infinitesimal isn't clearly defined in the tiering system, so there's not really a "threshold" for it
To answer your question in a more concise way: the character would be Low Hypoverse level and not Infinitesimal because infinitesimal is greater than zero, and this character is negative, which is less than zero