This species is distinguished by the following combination of characters: gill rakers 16-20 in upper series of first gill arch, 22-26 in lower series, 39-46 in total; gill rakers 12-14 in upper series of second gill arch, 21-26 in lower series, 33-40 in total; gill rakers 9-12 in upper series on third gill arch, 11-15 in lower series, 23-26 in total; gill rakers 7-9 in upper series on fourth gill arch, 10-12 in lower series, 17-20 in total; gill rakers 3-6 on hind face of third gill arch; prepelvic scutes 3-5, lacking spines; transverse scales 8; vertebrae 20 + 19-20 = 39-40; maxilla rather long, posterior tip just reaching, or slightly beyond, posterior margin of preopercle, slightly beyond anterior margin in individuals smaller than 40 mm standard length, 16.7-20.4% of standard length, 64.4-83.3% of head length; caudal peduncle short, 15.4-18.2% of standard length; no predorsal scutes; pelvic scute without spine; posterior border of preopercle convex, rounded; two pairs of dark patches on parietal and occipital regions without a following pair of dark lines; no black spots below eye and lower-jaw tip (Ref. 123745). Stolephorus zephyrus is easily distinguished from all other congeners, except for S. insignus, by a rather long maxilla extending posteriorly to, or slightly beyond, the posterior margin of the preopercle, the pelvic fin posteriorly reaching to vertical through the dorsal fin when depressed, no spines on dorsal-fin origin and pelvic scute, a convexly rounded posterior preopercle margin, no black spots on the suborbital area and lower-jaw tip; distinct paired dark patches on the parietal and occipital regions without following paired lines, and fewer than 27 lower gill rakers on the first gill arch (Ref. 123745). It can be distinguished from S. insignus by lower gill-raker numbers on the first gill arch, 16-20 + 22-26 = 39-46 in S. zephyrus vs. 19-21 + 26-28 = 46-49 in S. insignus; and a shorter caudal peduncle, 15.4-18.2% of standard length vs. 18.4-19.8% (Ref. 123745). Stolephorus zephyrus is easily distinguished from S. mercurius and S. rex due to the lack of paired pigmented lines on the dorsum of the former from the occipital region to the dorsal-fin origin; it further differs from S. mercurius in having a more slender body, body depth 19.5-24.6% of standard length in S. zephyrus larger than 40 mm standard length vs. 18.0-20.8 in S. mercurius, and lower gill-raker numbers on the first gill arch, 16-20 + 22-26 = 39-46 vs. 18-22 + 24-28 = 44-49 in S. mercurius; and from S. rex in having a shorter head, 23.9-27.5% of standard length in S. zephyrus vs. 24.3-27.6% in S. rex (Ref. 123745).

Description: Dorsal fin with 3 unbranched and 12-14 branched rays; anal fin with 3 unbranched and 17-19 branched rays; pectoral fin with 1 unbranched and 12-13 branched rays; pelvic fin with 1 unbranched and 6 branched rays; caudal-fin rays 19 (Ref. 123745).

Colouration: Preserved specimens: body uniformly pale, a silver or light ivory longitudinal band, sometimes lost, of width narrower than pupil diameter, extending from just behind upper opercular margin to caudal-fin base; pairs of dark patches on occipital and parietal regions without a following pair of dark lines; a few melanophores scattered on anterior parts of snout tip, but not on suborbital area; all fins pale; melanophores scattered along dorsal- and caudal-fin rays (Ref. 123745).