Helmeppo’s enlistment alongside Koby — the post-Ch. 7 Helmeppo joins the Marines too outcome, implied by the Ch. 7 page-17 Helmeppo + Lt. Ripper background composition, lands here as visual fait-accompli.
Both are at the literal-bottom Chore Boy rank — cf. the Ch. 66 SBS naval rank chart, where Chore Boy is the bottom rung below Seaman Recruit. The chapter’s title Luffy in Black + the Chore Boy cover-content is thematically paired — Luffy is being beaten as a captive in the chapter; Koby/Helmeppo are being trained as Marines on the cover. Two parallel under-the-thumb situations.
Both are at Branch 153 under Lt. Ripper — same lieutenant who was the 153rd Branch officer in Ch. 7. Their post-Ch. 7 supervisor relationship is canonized.
The cover-serial mode is the defeated-secondary-character cover-serial mode applied to reformed-secondary-character arcs — distinct from the Buggy recovering-villain cover-serial mode (Ch. 29 snowboarding / Ch. 33 HAPPY VICTORY). Oda is using cover-stories to track multiple off-page character lines simultaneously:
The Sanji cover-story (started Ch. 80) — future-Straw-Hat-in-cosmopolitan-travel.
The Koby/Helmeppo cover-story (started Ch. 83) — reformed-Marines-in-training.
Two cover-story arcs running in parallel is itself a new register install for the series.