Quests
Fire Emblem Engage puts almost everything optional on the world map between story chapters, which makes it easy to treat it all as filler. It is not. Some of it hands over units and Emblem Rings you cannot get any other way, and some of it is a story chapter you have to clear regardless. The useful sort is by what you lose if you walk past.
The side missions that give you people
Paralogues are the highest-value optional content in the game, because two of them are the only way to get two of your units. Jean comes from Budding Talent and Anna from Mysterious Merchant, and neither appears in a story chapter, which is why our companions guide cannot give either of them a chapter number.
The rest of the paralogue list is named after Emblem heroes rather than recruits: the Lucina, Lyn and Ike paralogues among them. Our page gives each one an unlock chapter and a recommended level, and the recommended level is the number to respect. A paralogue does not scale to you, so arriving five levels light is how a run ends on classic mode.
One of them, The Exalt, is the example our own page uses for structure: two objectives rather than one, destroying crystals as well as defeating the enemy commander. Read the entry before you deploy, because the deployment slots you get are the deployment slots you have for both tasks.
The event that gives you rings
Lookout Ridge is DLC content and it is the single densest reward in this bucket: one event, three Emblem bracelets. Edelgard, Dimitri and Claude all arrive together, plus the Tiki Starsphere skill.
It is here because the game does a poor job of telling you it exists. There is no obvious prompt, so people who own the DLC routinely finish the campaign without triggering it. Given that Emblem Rings are the scarcest resource in Engage, and that our inherited skills system runs entirely on having enough rings to rotate through your army, three extra bracelets is a bigger deal than three extra weapons would be.
The same applies to the other DLC drop covered elsewhere in this wiki: Wave 2 adds Soren, Hector and Camilla. Between the two, the DLC roughly doubles the number of Emblems you can be levelling bonds with at once.
The chapter you cannot skip
Chapter 21: The Return is the one story mission with a walkthrough of its own in this wiki, and it earns it. It takes you back to Elise, the site of the game's first defeat, and it is structured as a sequence rather than a single fight: clear the map, survive a wave of reinforcements, then three separate boss encounters in Griss, Zephia and Veyle.
Two things make it the chapter people look up. The reinforcement wave arrives after you think the map is won, which catches units that have already moved. And it is where Mauvier joins, automatically, at chapter 21, already ranked eighth of thirty-six on our tier list. A unit that good walking in at that point is the clearest signal the game gives that hoarding experience for late arrivals was the correct plan.
If you are working out what order to do any of this in, the rule that holds across all three pages is simple: paralogues before the story chapter they unlock alongside, because the units and rings they give you are useful in everything that follows, and there is no reward for finishing the campaign with them still on the map.