|Reinhardt isn't a formidable mage knight just because of his magic; he's an expert swordsman as well. This build aims to take advantage of his sword's unique effect and improve his performance on either phase.
Weapon: '"`UNIQ--item-202--QINU`"' is Reinhardt's weapon of choice. It's effectively a souped up '"`UNIQ--item-205--QINU`"', boasting three more Mt, but it also allows him to strike twice even on the enemy phase. That is to say, even if he gets attacked, he'll respond with two consecutive strikes just as if he initiated combat. This gives him an edge over other Brave weapon users who generally have no enemy phase presence due to their weapons only striking twice on the player phase.
Assist: '"`UNIQ--item-208--QINU`"' is a good Assist in general to use. Its uses are versatile, as it can either help bring an ally closer to the fray, or bring an ally out of danger. Cavalry units put it to great use with their high Mov.
Special: '"`UNIQ--item-211--QINU`"' is Reinhardt's Special, which lets him pierce through an enemy's Def. However, he needs to have the '"`UNIQ--item-214--QINU`"' Seal when using this skill, as he'd be able to activate it on the second strike. If the Seal is unavailable and Reinhardt is using a different Seal, then '"`UNIQ--item-217--QINU`"' would be preferred instead. It also lines up well with the rhythm of his attacks. He'll be able to strike twice and defeat an opponent, and his next victim will take two strikes, with the second hit having Luna charged.
A Passive: '"`UNIQ--item-220--QINU`"' is Reinhardt's A skill. With it, he'll be able to retaliate against mages and most archers, and thanks to his weapon's effect, he'll respond with two consecutive strikes. This is to ensure that more fragile ranged foes won't be able to take advantage of Reinhardt, and will have to think twice before engaging him.
B Passive: '"`UNIQ--item-223--QINU`"' is an option for his B skill, as it can help deter an opponent from charging their Special, letting Reinhardt safely take on units who rely on their Special. However, this skill slot is really flexible, as there are lots of other options he could put to good use. '"`UNIQ--item-226--QINU`"' is an interesting option, as he'll be able to strike first and strike twice when he's attacked while he's at a low enough health. This lets him get the jump on some foes and potentially surprise them. '"`UNIQ--item-229--QINU`"' lets Reinhardt retreat after attacking, helping him return to his allies and stay in better formation. '"`UNIQ--item-232--QINU`"' lets him debuff a tough opponent from afar, which either supports his team, or softens up his next target. '"`UNIQ--item-235--QINU`"' lets Reinhardt smash through sturdier sword wielders. Whatever he uses is up to the player's discretion, as he should be fine with anything.
C Passive: '"`UNIQ--item-238--QINU`"' is an option for Reinhardt's C skill, letting him augment an ally's Atk and Def, making them sturdier. However, this skill slot is flexible, as there are few other options he could use. Cavalry buffs like '"`UNIQ--item-241--QINU`"' or '"`UNIQ--item-244--QINU`"' could work, or even other regular buffs like Tactics skills could be used if he's not on a cavalry team. '"`UNIQ--item-247--QINU`"' could help Reinhardt pierce through more opponents. After any round of combat, whether he's initiating or retaliating, any nearby foe will suffer a Def debuff, letting him deal even more damage to them. Whatever he uses here is up to the player, as anything should be fine.
Sacred Seal: '"`UNIQ--item-250--QINU`"' is necessary as Reinhardt's Seal if he is using Moonbow, as it helps bring the charge down to 1 so that he can activate it in his first engagement. Should it be in use or is unavailable, then a simple Atk boost with '"`UNIQ--item-253--QINU`"' will help maximize his power on either phase, while '"`UNIQ--item-256--QINU`"' will help improve his enemy phase power, especially if he has '"`UNIQ--item-259--QINU`"' to get the jump on his opponents.