The Looper (Fortnite Battle Royale) vs Billy Kid: who would win?

Billy Kid favored — Slight edge (~55%)

The Looper (Fortnite Battle Royale) · Fortnite · Base

Tier
8-C, up to 8-B
Attack Potency
Building Level, up to City Block Level
Speed
Average Human, up to Supersonic+
Durability
Small Building Level+, up to City Block Level

Billy Kid · Zenless Zone Zero · Base

Tier
8-C
Attack Potency
Building Level
Speed
Superhuman, up to Massively Hypersonic
Durability
Building Level

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