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Petra is one of the premier speed demons in Three Houses. She has one of the highest base Speeds of all the students at 10, and has a very reliable 60% Speed growth to back it up. Like most Fire Emblem games, Speed is an extremely important stat, and Petra will be one of your first units to be able to double enemies.

Like many other units, Petra will perform excellently in either of the Flying master classes. She already has enough Flying rank at base to certify for Pegasus Knight, and her Lance rank can be trained to D+ with very little instruction investment. It is worth it to first master Brigand before changing into a flier, since Death Blow will help her deal enough damage when attacking. After gaining Darting Blow, she can continue on into Wyvern Rider and then either Wyvern Lord or Falcon Knight, depending on player preferences. As either Flying master class, Petra will have both Avo +10 and Alert Stance+. With her blistering Speed stat, most enemies will probably have single digit displayed hit on her if she waits during the player phase.

Giving her the Chalice of Beginnings (DLC only), or using the Retribution gambit on her will allow her to dodge tank on enemy phase while dealing significant damage back. In particular, this can be useful for removing siege tome enemies, since Petra does enough damage to one round them with Counterattack.

Wyvern Lord is usually the preferable class in most cases. She will be somewhat faster as a Falcon Knight, but Petra is already so speedy anyways, and she really appreciates the added Strength that Wyvern Lord gives. Petra has a boon in Axes, and will also learn some interesting combat arts when ranking up her Axe skill (she does not learn any Lance combat arts, aside from the normal ones). In particular, Wild Abandon when used with a Killer Axe along with a Critical Ring will often approach around 80% critical chance, thanks in part due to Petra's high Dex.

Petra's damage does start to fall off a little towards the end of the game on Maddening mode, since she usually won't be able to double (and therefore one-round) the faster enemies such as Assassins and Falcon Knights. Her lack of a Crest means that she can't wield any Relics reliably, and will have to rely on generic powerful weapons to deal enough damage. At this point, she is better off as a pure dodge tank, serving to soften up enemies during the enemy phase for your other units to defeat on the player phase. Enemy Dex stats never get high enough to reliably hit her with all of her Avoid abilities active. Of course, Petra can still do very decent chip damage, assuming she doesn't need to be dodging the next enemy phase.

Certifying and mastering Assassin (even just over the course of some Auxiliary battles) can also be a good option. Petra will easily reach around 40 Dex before the game ends, giving her a fairly relevant 10% chance to proc Lethality. Being able to one-shot enemies like this becomes more and more relevant as the game goes, especially since it can also proc on the enemy phase. Petra can be send to dodge tank a big group of enemies, and she will probably end up killing a handful of them with Lethality while softening up the rest. Petra has boons in both Swords and Bows, so she should be able to easily class into an Assassin after some instruction. Once she masters Assassin, she should immediately switch back to a Flying class.

Overall, Petra is an excellent unit to use, both in and out of house. She's one of the few students who has a decent early game, and will grow to become a very speedy dodge tank who can fly into the middle of the enemy army without any fear.

Petra can build support with the following characters.

Petra will automatically be recruited if the player selects to teach the Black Eagles house.

Petra can be recruited once Byleth has achieved a stat of Dex 10 and Skill Rank C in Riding. These requirements can be lowered if Byleth builds support with Petra, as shown in the below table.