Characidium satoi is distinguished externally from its congeners by having area between pelvic fin and origin of anal fin moderate to strongly convex in lateral view (vs. area between the pelvic fin and the anal-fin origin straight); and by color pattern, with females, juveniles, and non-breeding males consisting of irregular and discontinuous bars, forming dorsal blotches separated from ventral part, which is V-shaped, W-shaped, or diamond-shaped, or breeding males having less distinct patterning and an overall darker, more uniform pigmentation. It possesses one putative internal autapomorphy, the anteriormost first anal-fin radials inserted between the fifth and sixth caudal vertebrae. Additional external diagnostic characters useful for its identification include the following: isthmus scaled; lateral line complete, with 34-37 perforated scales; anal-fin origin far posterior from anus, separated by 4 to 7 scales (vs. anal-fin origin close to anus, separated from anus by 2 to 4 scales in all congeners, except in C. hasemani, C. litorale , and C. zebra ); distal tip of anal-fin rays reaching base of ventral caudal-fin ray (vs. not reaching, except in C. bahiense, C. hasemani, C. mirim, and C. zebra, and mature males of C. rachovii); and presence of hooks on the pectoral- and pelvic-fin rays in males (vs. absence, except in C. bahiense, C. bimaculatum, C. lagosantense, C. litorale, C. mirim, C. nupelia, C. stigmosum, C. xanthopterum, and C. xavante) (Ref. 104008).

Description: Dorsal-fin rays ii+9; anal-fin rays ii+6; pectoral-fin rays iii+7+i, iii+8+i, iii+8+ii, or iii+9+1; pelvic-fin rays i+7+i (Ref. 104008)..