Information about how to recruit Petra in Fire Emblem Three Hopes.
In Fire Emblem Three Houses you could recruit many characters simply by getting specific stats and skills high enough. Once done, they’d be impressed enough to realize that studying under you would suit them better than petty considerations such as life-long friendships, national loyalty, familial duty or even religious conviction.
Unfortunately characters in Fire Emblem Three Hopes aren’t quite as malleable - some characters will join your faction as you progress the Main Questline, usually due to story contrivances entirely outside of your control. The rest, however, must be met in battle and “persuaded” to join you… often by lumping them on the head enough times to realize that they’d be better off defecting than decomposing.
Character Recruitment Strategies
In the case where a character can be persuaded to join you, this “persuasion” effort is achieved via the deployment of a Strategy. When you start a battle in which a character can be recruited you’ll first have to pick from a variety of available [Strategies]. You can find more information about Strategies on their own page, but for the purposes of recruiting characters you’ll be interested in any “Persuade [Character]” Strategies, which you must select prior to the battle to ensure you can utilize said Strategy(ies) during the battle. Deploying Strategies in turn require you to expend Strategy Resources, a currency gained by completing [Side Quests] during the chapter leading up to the Main Quest where enemy characters can be recruited.
During battle, when the character you wish to recruit spawns you must activate the Strategy to persuade them by entering the battle menu before you defeat them. If you defeat them without activating the Strategy - even if you selected it before the battle - they will die and you will not be able to recruit that character for the rest of the playthrough! Once the Strategy has been activated, the area where the Strategy is in effect (usually where the target character spawned) will be surrounded by a purple glow. Merely defeat them in this area and you’ll recruit the character.
How to Recruit Petra in the Scarlet Blaze Route
Due to good, old-fashioned, heavy-handed politics, Petra is a guest (hostage) of the Adrestrian Empire. She’s available from [Chapter 3] on provided you opt to join Edelgard and the Black Eagles.
How to Recruit Petra in the Azure Gleam Route
While not immediately under the influence of the Kingdom of Faerghus, Blue Lions players don’t have to wait long to convince Petra to shift her allegiance. During the [Main Quest in Chapter 4] you can deploy the Persuade Petra Strategy by expending 10 Strategy Resources - a paltry sum, considering you need to clear at least two Side Quests to begin the Main Quest, netting you 20 Strategy Resources along the way. Play through the battle normally and when Rhea gets near the northeastern corner of the map, Petra will spawn and attack. Activate the “Persuade Petra’’ Strategy and defeat heir to Brigid, after which she’ll see reason and choose defection over destruction - better to be the “guest” of the Kingdom of Faerghus, anyways, everybody knows blue is more trustworthy than red.
How to Recruit Petra in the Golden Wildfire Route