Ruby (Crystal Gems) vs Saiko Yonebayashi: who would win?

Ruby (Crystal Gems) favored — Slight edge (~55%)

Ruby (Crystal Gems) · Steven Universe · Base

Tier
8-A
Attack Potency
Multi-City Block Level
Speed
Relativistic
Durability
Multi-City Block Level

Saiko Yonebayashi · Tokyo Ghoul · Base

Tier
8-A, up to LOW 7-C
Attack Potency
Multi-City Block Level, up to Small Town Level
Speed
Massively Hypersonic, up to Massively Hypersonic+
Durability
Multi-City Block Level, up to Small Town Level

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