Ryūji Saeki holds the position of A-rank All-Rounder in Border's Kusakabe Squad. He sports black hair parted in the middle and swept to the left, standing slightly shorter than squadmate Kazuma Satomi. His uniform features a high collar, primarily black with olive-drab sections on the biceps and chest pockets, accented by white stripes along the sides and zipper lining; the Kusakabe squad logo sits above his left chest pocket.

Saeki exhibits a forthright personality, openly voicing opinions and questioning squad leadership when he deems it necessary. This extends to enthusiastic commentary on other agents' cooking skills, revealing a strong interest in home economics. Outside this focus, he maintains a generally serious yet relaxed demeanor. His documented preferences include praising seniors, soccer, curry, and shōnen manga.

His background indicates prior leadership experience before joining Kusakabe Squad. Character profiles suggest he could have led "Saeki Squad," though complications arose from Kusakabe Squad's rapid advancement. Conflicting accounts note he previously commanded his own unit. Jin identified Saeki as one of twelve candidates eligible to inherit the Black Trigger Fūjin during the Border Enlistment Arc.

Throughout the B-Rank Wars Arc, Kusakabe Squad, alongside Katagiri Squad, conducted scouting operations for potential recruits outside the prefecture. Saeki briefly appeared with squadmate Hayato Uno before the Final Round, calling out to Satomi.

During the evaluation phase of the Away Mission Test Arc, Saeki assessed squad dynamics, noting Yuba Squad would maintain tight discipline in Phase 1. He questioned Captain Kusakabe about accommodating Kuruma Squad's relative slowness in Phase 2, emphasizing a cautious strategy. In Phase 1, he reacted enthusiastically to meals prepared by Ikoma Squad and Kirie Konami, expressing admiration for their cooking skills and a desire to sample the food.