In Fire Emblem: Three Houses, the players start by choosing between the Black Eagles, Blue Lions, and Golden Deer houses to teach and lead in battle. While each house automatically comes with a well-rounded set of students, it is possible and highly encouraged to recruit other units to join your side.
However, convincing other students and faculty to join your side is not always easy. Some units are harder to impress than others, depending on what skills, stats, or levels they gravitate towards. Thankfully, there are a handful of recruitable units who join your team with little to no effort.
10 Alois
Alois is one of the Knights of Seiros who has a close relationship with Jeralt. As a man in armor, his intended path seems to lead to becoming a Great Knight, though he makes a good War Master as well if recruited.
To recruit him, all you need to do is be level 15 or higher and talk to him from Chapter 11 on. The only inconvenient part is that there's only a two-month window to make sure he's recruited before the time-skip.
9 Felix
At first glance, Felix seems like a picky character to recruit. He is the only student who bases his recruitment around Byleth's Speed and Swordsmanship, as well as the only one who needs Byleth's skill to be at a B+ rank.
However, this is actually more convenient than it sounds since Byleth has a decent growth rate in Speed and generally wields a sword as their main weapon. This means the professor can stick to their strengths while attempting to impress Felix.
8 Hanneman & Manuela
Hanneman and Manuela are the other two professors at the Officers Academy who are in charge of teaching the other houses. There is a friendly rivalry amongst the professors that is most notable during the mock battles.
Coincidentally, after the mock battle at the Field of the Eagle and Lion, both Hanneman and Manuela can be recruited at the same time as long as the player is at least level 15.
7 Caspar
Caspar is a feisty kid who will always stand up for what he believes in. If necessary, he will teach wrong-doers a lesson with his fists.
Therefore, in order to impress Caspar, it's important for Byleth to be both physically strong and skilled in Brawling. This shouldn't be a problem since Byleth has a natural affinity for Brawling, though only the male version has access to the Brawler class line.
6 Shamir
Shamir might come off as cold and distant based on her short conversational responses, but she's easier to win over than she lets on.
Like most of the other knights and faculty members, Shamir will be recruited if Byleth is level 15 or higher starting in Chapter 6. She can prove to be an early asset as a bow expert with high damage output.
5 Catherine
Known as "Thunderstrike Cassandra," Catherine bears the Crest of Charon along with its linked Hero's Relic, Thunderbrand.
Catherine can join the player's team as early as Chapter 4 when Byleth achieves level 15. Regardless of whether or not you want to use Catherine as a unite on the battlefield, it's a good idea to recruit her as soon as possible to snag Thunderbrand.
4 Dorothea
Dorothea is an opera singer with a lot of charm and a playful personality. Naturally, she would want to learn from a professor who is equally charming.
To recruit Dorothea, Byleth needs a high Charm stat and Authority. While Authority is already one of Byleth's strong suits, Charm can easily be raised by partaking in tea time. Inviting Dorothea to tea is the perfect way to raise both Byleth's Charm and Support with the songstress.
3 Ignatz
Ignatz is a smart and open-minded student who is one of the easiest students to convince to transfer into your class. In terms of stats and skills, he doesn't have any extreme demands for Byleth before he's sold.
All Ignatz asks is for Byleth to have at least 10 Dexterity and a B rank in Authority. It is very possible for Byleth to fulfill these requirements naturally without even trying due to their growth rates in Dexterity and a knack for Authority.
2 Cyril
Cyril is neither a student or faculty member, but he resides at the monastery to help out the Archbishop in any way.
Like faculty members, Cyril's recruitment is based on level. However, he only asks for Byleth to be level 10 rather than 15. This makes him one of the earliest possible recruits in the Blue Lions and Golden Deer routes.
1 Sylvain
By nature, Sylvain chases after any and all females, including Lord Gwendal's daughter and Ingrid's grandmother.
Therefore, he will immediately gravitate toward the player if they choose to play as female Byleth. Throwing all of his requirements and expectations out the door, Sylvain is by far the quickest unit to be swooned by female Byleth. On the other hand, if Byleth is male, Sylvain will require decent Charm and Reason, which are still fairly reasonable to obtain.