|Weapon: Raijinto is Ryoma's weapon by default and is one of the best weapons in the game, for it has Distant Counter built into it. It is also integral to this build because it allows him to counter all attacks while still amassing special charge from Steady Breath. Being able to counter at all ranges ensures that he will also be always able to activate his special when it is ready.
Assist Skill: Reposition is an excellent assist skill for many units and Ryoma is no exclusion to this. Reposition allows for additional maneuverability in any team. For example, he could reposition an ally behind him and then become a powerful wall for that ally thanks to his equipped Steady Breath.
Special: Moonbow is chosen for its low charge time and damage boost. When an enemy attacks Ryoma, Steady Breath will push the cooldown from 2 -> 0, meaning Ryoma immediately retaliates with a Moonbow attack proc.
A Skill: Steady Breath is by far the most important element to this build. Just about every other skill in this build is geared towards coordinating with Steady Breath. This skill not only grants Ryoma extra bulk while he's under attack but essentially supercharges his special. This skill permits him to activate Moonbow immediately after the enemy's first attack. When under attack, Ryoma recieves +4 defense, and this defense buff contributes to the damage done by his special (and this is not including any buffs from teammates).
B Skill: Quick Riposte 3 provides an extra element of synergy to this Steady Breath build by enhancing his dangerous presence during the enemy phase. Coupled with the Raijinto, he will always double units that attack him, regardless of both range and speed, given that he is within a healthy HP threshold.
C Skill: Hone Spd 3 is what Ryoma comes with naturally, so it's an option for his C skill. His allies could appreciate a Spd buff, but this slot is ultimately versatile. There are not many skills that can help this build directly, so any skills that could help Ryoma's surrounding teammates would be advisable for this slot, like his natural Hone Spd, but other buffs can be used if needed.
Sacred Seal: This slot is also relatively versatile in terms of what would be optimal. A good defensive example would include Distant Def 3 for instance.