I want to make three broad types of units: support, who aren't much good at dealing damage but provide rallies, healing, and other supportive skills; attackers, who deal high damage, and tanks, who have high defense.
1. Kana: Attacker. Father will be Izana (making him a good support unit).
Kana favors skill, speed and luck. From my avatar, my Kana will have access to the standard Nohr Noble stuff, as well as Samurai. This grants him Astra, Swordfaire (goes well with Nohr Noble), Life and Death, and Vantage.
With Izana as a father, Kana's the only kid who can have access to the Onymoji and Great Master class, too.
Friend: Siegbert (Cavalier) or Percy (Wyvern Rider).
Final skill set: Vantage is a good start. Astra, Dragon Fang, Draconic Hex, Life and Death, Nohrian Trust, and Swordfaire/Tomefaire are other good choices.
2. Shigure (Azura/Laslow): Support Unit. Will be a rally unit thanks to his personal skill. Ideally will be a berserker for as long as possible to grab as much HP as possible so that his personal can be of use. If I ever get the grandmaster class, it naturally goes to Shigure.
Married: Velouria (for Rally Strength).
Friend: Forrest. Doesn't give him anything, but Shigure has no other choice.
Final skill set:
Rally Speed (Falco Knight), Rally Strength (Berserker), Rally Skill (Bow Knight), Rally Resistance (Strategist), Inspiration/Amaterasu (Strategist/Kinshi Knight).
3. Dwyer (Jacob/Nyx): Support Unit. Dwyer will be my end-game Butler. It's thematically appropriate, and his mediocre strength prevents him from being a front line character anyway. Either that, or I make him a mage...but then I'd have to find another Butler.
Married: Nina...? Not Ophelia, she gives the same class as Dwyer's mother. Either Nina or Soleil, but Dwyer's already got a healing skill.
Friend: Percy (Wyvern Rider).
Final skill set: Inspiration, Seal Magic, Pass, some breaker skills?
4. Midori (Kaze/Beruka): Attack Unit. Have Kaze pass Astra (or vantage?) down to Midori; she can't get it normally. Spendthrift has established itself as a devastating skill due to the ease with which gold bars may be farmed; Midori will try to give that to as many people as possible.
Midori is one of two female characters that can get Vantage (through Kaze), making her a good candidate for the Great Lord class (with Awakening). With her mother as Beruka and her good skill growth, she could work nicely with Aether.
Married: Midori grants two extremely powerful classes, mechanist and merchant, which give replicate and spendthrift respectively. She should marry a unit who is tough enough to withstand damage from being in two places at once. Ignatius, perhaps, or Siegbert.
Friend: Ophelia (Dark Mage).
Final skill set: Profiteer (initially, will be dropped), Spendthrift, Replicate, Trample, Savage Blow, Astra, Awakening, Aether,
5. Sophie (Silas/Selena): Tank or Attack Unit. Sophie's one of my tougher units, and with her defenses covered for the most part it might be fun to explore more offensive options with her. Currently a Great Knight to pick up Luna and Armored Blow, both useful skills. Hero with its Sol is useful too...Axebreaker might not be needed but is a nice pick-up. Bowknight's rally skill is kind of iffy, I don't want her to be a background support character, but I suspect that Shurikenbreaker will be highly useful. Selena grants her the falco/kinshi knight classes...warding blow could be a nice complement to armored blow, making her good at attacking anyone as long as she can survive the following turn. Selena gave Sophie the interesting choice Sky Knight; she is the only child who can use it.
Her crappy mag growth makes her a poor pairing for Dwyer or Forrest, though I suppose Maid has its useful skill Tomebreaker. Sophie's resistance is passable, though.
Married: Sophie's not really lacking anything; I might not get her married at all. Percy/Siegbert/Ignatius if I have to marry someone.
Friend: Velouria (Fighter).
Final skill set: Luna, Armored Blow, Axefaire, Axebreaker, Sol, Warding Blow, Air Superiority, Trample, Savage Blow.
6. Siegbert (Xander/Charlotte): Attack Unit. Kind of like Sophie right now, which is unsurprising since they're both Great Knights (I really don't like the paladin class). Siegbert is geared up to be the epitome of physical strength thanks to his mother. The Wyvern line offers a bunch of good offensive skills. The Berserker/Hero line also help increase his survivability and damage, particularly with axes. Given his high offensive power and toughness, he's a good candidate to marry Midori and take advantage of the replicate skill...tentatively. He has some really interesting marriage/friendship sets which I need to think more about.
Final skill set: Pavise, Aegis, Replicate, Armored Blow, Axefaire, Axebreaker, Sol, Luna, Trample, Savage Blow, Tomebreaker
7. Forrest (Leo/Felicia): Attack Unit. Forrest has the best mag of any unit in my army right now, so skill-hunting will be used to promote his survivability as top priority, followed by ways to make him even more destructive. He should be in the Sorcerer class for as long as possible to maximize his magic growth. Forrest's high magic growth and terrible everything else (outside of resistance) means he'll one shot someone, hopefully hitting, but he's not good for much else outside of possibly being a mage tank.
Since Forrest is so prone to taking gouts of damage due to his terrible defence, it's tempting to make use of vengeance. However in most circumstances I've seen he simply dies before he gets to take advantage of this skill. In light of this, I want to focus on grabbing as many breaker skills as I can. The Great Knight's armored blow, General/Paladin's pavise and aegis, and Hero's Sol are all good choices, but Forrest will want to get out of these classes ASAP due to Forrest's horrible strength.
Something worth considering is that Forrest has atrocious skill growth. Probably put fewer activated skills on him if possible.
Friend: Ignatius. Literally all four skills Ignatius gives Forrest are useful. His weak speed will be covered by Wary Fighter, pavise might save his life despite its low activation rates, armored blow will minimize reprisals if he misses (and he'll miss often).
Final Class and Skill set: Sorcerer, Bowbreaker, Lifetaker, Inspiration, Shurikenbreaker, Axebreaker, Pavise, Wary Fighter, Armored Blow, Luna.
8. Ignatius (Benny/Peri): Tank. My Ignatius, in his current state at least, is nice and unkillable physically. This could make certain skills like armored blow redundant. I COULD go all the way with all the defensive skills and make him unkillable, or I can pick up some offense if I friend him with Percy for the wyvern line, and marry him to someone like ophelia to introduce some more offensive skills. Ignatius' defense is already off the charts, so there's little point in keeping him in the general class for more def. Consider reclassing to Hero to maximize his skill growth (for the highest chance of Aegis/Pavise activation) or something that grants him good resistances.
Friend: Forrest (Troubador). Tomebreaker seals off his last remaining weakness unless he gets extremely unlucky (he already uses lances which have an advantage against tomes in the first place). Inspiration might be nice.
Final Class and Skill set: General, for the high defense cap, but it might be unnecessary. If he can survive being a great knight it'd give him increased mobility.
9. Nina (Niles/Mozu): Attack Unit. Thanks to Mozu's aptitude, Nina has had amazing growths practically across the map. She is still rather fragile, however.
Her mom being Mozu, Nina has some interesting options other Nohr children don't have. She can pair Life and Death together with Spendthrift to generate a whopping +20 damage. From Niles, Life and Death goes well with Vengeance. She gets a couple of useful breaker skills as well. She can marry Percy to further expand her devastating offensive potential, or she can marry the others for a mix of offensive and defensive skills. Armored Blow from Great Knight would nicely counteract Life and Death's drawbacks.
Friendship: Velouria (Fighter).
10. Ophelia (Odin/Elise): Attack Unit. Alt class is Samurai. Duellist's Blow and Vantage are useful also, and they're still good choices.
My friend makes an interesting case for Ophelia to pick up Vantage, Astra/Vengeance, Life and Death, and friendship set her with Midori and grab Spendthrift. This way, she can hypothetically deal a +50 damage if she activates Astra with a gold bar on hand. I could either go vantage/vengeance, or lifetaker/something else to complement this combo.
Ophelia is the only child character in Nohr who can access the Samurai class, giving her vantage, vengeance, etc. She is a candidate for the great lord class, allowing her to get Awakening...Awakening, Vantage, Life and Death, Vengeance and Spendthrift sounds like a pretty devastating combo.
Friend: Midori...or Soleil? Ophelia's personal skill clashes with Spendthrift (for inventory management). However, Replicate's good, and Ophelia's great luck growth makes her a good candidate to get Profiteer with. On the other hand, Soleil gives Sol, Shurikenbreaker and Axebreaker. Breaker skills are nice for a pretty frail character.
Final Class and Skill Set: Sorcerer, with its high magic cap and high rank in tomes, is a natural fit for Ophelia.
11. Percy (Arthur+Effie): Attack Unit.
Percy's poor strength growths are nicely countered by the fact that his mom had ridiculous amounts of strength that she passed down.
Friend: Kana. Ignatius gives the same skills Effie did, and Dwyer's mostly support skills.
Final Class and Skill Set:
12. Soleil (Laslow+Azura):
Friend: Soleil's kind of squishy, so maybe she'd benefit from Armored Blow from Sophie (especially with the replicate skill, this'd help her preserve her life). Luna's a good choice because Soleil's mum is Azura and therefore she benefits from high skill. Aegis would further increase her res, though her res is not bad by default. Ophelia gives a bunch of great skills though...Vengeance, Bowbreaker, Lifetaker and Seal Magic all have their uses.
Final Class and Skill Set: Master Ninja. Good innate bonuses, and she's the only one who can use some of the end-game hidden weapons. Replicate, Sol/Luna/Lethality, Lifetaker, Shurikenfaire, a bunch of breaker skills (including air superiority). Soleil's one of the few people who can make good use of Lethality. WIth a soldier's knife and replicate, she has 8 chances to activate it per turn, and with her good skill this isn't outside the realm of impossibility.
A good final combo to settle on is soldier's knife/caltrop. Park Soleil at the end of a turn behind some allies with a caltrop handy. The enemy will get support attacked by soleil, causing them to have big debuffs the following turn. Then attack with soldier's knife dealing huge damage; end with one of the soleils debuffing a new target with caltrop (as her damage post-attacking with soldier's knife would suck anyway).
13. Velouria (Keaton+Camilla): Attack Unit. Velouria can get Vengeance or Lifetaker from Camilla, both of which are very useful for her given that she'll end up with a strong HP base.
Friend: Sophie. It bugs me that I keep picking Sophie as best friend--but Luna, Aegis, and Armored Blow are all excellent for Velouria. She's made for wading into the thick of battle and mauling hapless horsemen, and these all help her do her job better and/or survive the following turn. Nina's Pass and Shurikenbreaker are tempting...but shurikens won't do much damage to Velouria at all owing to her great defense boost from beast items and her solid HP pool, and Pass is only very circumstantially useful.
Final Class and Skill Set: Wolfseggner (or however you spell it): great stat boosts and beastbane make this an obvious final choice. Trample's a superb skill for Velouria and pairs well with beastbane (practically every mounted unit gets hit by beastbane; everyone else gets hurt by trample). Other good skills include savage blow, luna, armored blow.