|Being the first colorless melee unit in the game, Robin(FV) can be made to be a jack of all trades unit with fairly decent player and enemy phase. She has the flexibility to fit into most teams and can be kitted to handle specific threats due to her colorless nature. However, because she cannot have triangle advantage, she doesn't necesarily accel against any particular enemy type either.
Overall, the best boon/bane set for her is +Spd -Res as it helps focus her best boon, being fast, and takes away from her already mediocre resistance. Though, depending on what you want your Robin to do most boon/bane sets are adequate, but be wary of the def superbane.
Her weapon of choice is '"`UNIQ--item-376--QINU`"', the weapon she comes with. Being a better version of '"`UNIQ--item-379--QINU`"' it has more MT than an atk refine without any of the cooldown penalties. This allows Robin to reliably get a special attack off while only taking one hit.
'"`UNIQ--item-382--QINU`"' is generally the best assist skill, especially on fliers, as it allows for unparalleled movement options for your team.
'"`UNIQ--item-385--QINU`"' provides a good way for Robin to increase her damage output every combat. This is especially important for Robin, as she cannot have triangle advantage over units, and thus cannot abuse it in order to easily charge up a special to unload the next turn.
Her native A skill of '"`UNIQ--item-388--QINU`"' is a strong option for her to run. Being '"`UNIQ--item-391--QINU`"' and Bracing Stance combined into one skill, it provides utility against Bows and Excalibur, while still letting her get some defense out of it.
Alternately, one can opt for just using '"`UNIQ--item-394--QINU`"', which is a very strong option for her. If you do not need her to deal with Archers, '"`UNIQ--item-397--QINU`"' bolsters her speed and atk at minimal loss to her defenses to turn her into a powerful mixed phase unit. Having the stats permanently gives her more flexibility in the types of fights she can take.
While Robin is fast, she isn't able to consistently outspeed the fastest units, which makes '"`UNIQ--item-400--QINU`"' the best B skill for her whether running a full enemy phase build or a mixed phase build.
Alternately, if one opts to use the '"`UNIQ--item-403--QINU`"' Sacred Seal, '"`UNIQ--item-406--QINU`"' can be very useful to mitigate special attacks from enemies.
Robin is also unique because she can run every breaker except for '"`UNIQ--item-409--QINU`"', meaning depending on the rest of your team composition you can choose any breaker to help fill any gaps in coverage. This makes Robin very flexible in terms of team building.
While Robin's native C skill in '"`UNIQ--item-412--QINU`"' is a strong budget option, any other buff for the team such as '"`UNIQ--item-415--QINU`"', '"`UNIQ--item-418--QINU`"', or other generic buffs are recommended depending on your team composition.
Robin's Sacred Seal choice is also pretty open. To increase her ability as a melee duelist, '"`UNIQ--item-421--QINU`"' can be very useful. '"`UNIQ--item-424--QINU`"' is also useful if running the '"`UNIQ--item-427--QINU`"' build, as it helps further increase her spd to bolster her player phase.
If you chose to not run '"`UNIQ--item-430--QINU`"' as a B skill, you should run it as the seal instead.