Hero (Dragon Quest IX) vs Leo Kaiser: who would win?

Leo Kaiser favored — Slight edge (~55%)

Hero (Dragon Quest IX) · Dragon Quest · Base

Tier
4-A
Attack Potency
Multi-Solar System Level
Speed
Massively Ftl+
Durability
Multi-Solar System Level

Leo Kaiser · Saint Seiya · Base

Tier
3-C, up to LOW 2-C
Attack Potency
Galaxy Level, up to Universe Level+
Speed
Speed Of Light
Durability
Unknown

A heuristic estimate from normalized VS Battles Wiki stats: tier, attack potency, speed and durability.