My particular avatar has oni savage as a secondary and has married Anna. I just really want Anna to have Salvage Blow. As he has access to literally every Nohr class, his options are too flexible to come up with one definitive way forward.
The other good option for future runs include Spear Fighter, which is the *only* other class which grants classes not normally available through friendship or partner seals.
Felicia favors magic (less from great magic growths and more because of her terrible strength), resistance, and luck. Her middling HP growth means that she's not a good tank. Overall, she's probably best used as a support unit.
She can get Maid, Strategist, Hero, and Bow Knight. There's a good set of breaker skills here, though Tomebreaker's probably redundant due to her pretty good res. Strategist suits her well with Inspiration and Rally Resistance.
Her friendship sets are Flora and Peri. Both Flora's classes give Felicia something useful (Vengeance, Bowbreaker, Seal Magic, Lifetaker). Peri gives Luna and Armored Blow, which is good if Felicia's offensive potential happens to develop more than average, and Paladin, which gets Aegis. I don't think Aegis is super important for Felicia.
One advantage of Felicia is that she's a pre-promote and has a level cap of 40. That means she has PLENTY of time to get all the skills she needs.
Elise is characterized by insanely high magic and luck, good speed and resistance, middling skill, but terrible HP and defense. This makes her a very strong, but rather inconsistent glass cannon. She can pick up a few breaker classes to help with this. Malig Knight also grants her increased offensive power. Elise benefits from anything that gets her a higher chance to hit (Lucky Seven, for example). If her skill gains are good enough, Elise can marry Odin to gain access to powerful magical classes.
Elise's high magic potential makes her a good candidate for the Witch class.
Elise's only logical friendship choice is Azura. Camilla gets her literally nothing; Effie gives the knight class which as practically nothing good to offer her (her horrible defense and HP means that she's kind of screwed if anyone physical hits her, anyway). The only skill worth considering from Effie is Luna, which is only ok at best due to Elise's low skill growth.
Silas is solid all-round, with above average to average stats in most categories except resistance and magic. He performs ably in both offensive and defensive capabilities. His lack of weaknesses and good defense means that he's a good candidate for using Shelter to rush in and save vulnerable teammates.
Silas' solid defense and skill growth makes him a good consideration for tanking; he can get both pavise and aegis if he marries Effie, and they'll activate fairly often.
SIlas' friendship options are Kaze and Jakob. Jakob's Strategist line is pretty weak for Silas due to his terrible magic; however, Tomebreaker from Butler covers Silas' only significant weakness.
Effie is defined by her superb str growth, along with above average growths in every category save Magic and Resistance. Her classes grant her Armored Blow and Luna, both good; Pavise is a nice pick up as well, though she'll generally shrug off most forms of physical damage without too much trouble. Wary Fighter is a waste usually; her speed growth, while not too high, means she usually doesn't get doubled anyway. Her secondary is bizarrely Troubador. Strategist gives nothing too good except maybe inspiration if I can't figure out what else to give her. Maid gives her Tomebreaker, covering one of her only weaknesses nicely.
Effie can friend Mozu or Nyx. Mozu doesn't give much useful here; Effie has no use for the sniper class, and Kinshi Knight only improves her hit and dodge, which she doesn't need. Nyx, on the other hand, grants her seal magic, lifetaker, and vengeance. These are all potentially excellent skills for Effie to have.
Effie's marriage options are quite interesting. Marrying Xander or Silas gives her the paladin class, giving her Aegis which can further shore up her magical defenses. She could also get Hero from a bunch of people for Sol, which could be nice. Marrying Kaze would create one hell of a OHKO ninja, which might be worth considering.
Poor Arthur has the worst luck growth in the game. Basically, ANY level you get with him that has a luck stat +1 is a great level--even if you get nothing else. Arthur's terrible luck means he'll have poor hit rate and a dangerously high chance of getting hit and criticaled. With the exception of his weak resistance and awful luck, however, the rest of his stats are quite good, with superb HP, Str, and particularly skill, along with average defense and speed. Arthur is well suited to the Hero class both thematically and because of its access to swords, which are more accurate. He will activate Sol often to recover some of the HP he's bound to lose. Berserker will grant him even more damage output. Arthur's secondary, cavalier, is excellent for him. Armored Blow, Luna, and Aegis are all excellent.
Arthur can A+ Niles or Keaton for the Outlaw class, granting him Lucky Seven, which is perfect for him. Shurikenbreaker's also nice.
Mozu, Beruka, Camilla, Effie, and even Nyx are good pairing choices for Arthur. His high skill, HP, and the fact that he's bound to get hit some time make him an excellent pairing with vengeance (bowbreaker's another sweet bonus); Effie gives him general and thus pavise; Beru/Cam's Wyvern line gives swordbreaker, trample, and savage blow, all great. Mozu gives him certain blow, taking care of all his accuracy problems when he attacks.
Odin has the unusual trait of having manageable strength AND magic. His average growths throughout mean that he has great but inconsistent potential. Ditch him if he turns out badly. If he turns out well, though, he has many options.
Odin's skill is pretty good, so if it develops well he can grab astra and/or vengeance. His generally poor defense and res can mean he'll benefit greatly from lifetaker. His access to life and death makes him a good candidate for being a purely offensive unit.
He has surprisingly bad resistance, meaning that Leo granting him butler through friendship is not a bad idea. If his hit rate turns out to be awful he could friend Niles instead for Lucky Seven. Friending Laslow instead gives shuriken breaker, which could be nice, Axebreaker (generally unneeded due to axes' low hit rate and weapon triangle advantage with swords/tomes), and Sol. I'm not a fan of Sol on Odin despite his nice skill growth; lifetaker is more reliable for a high damage character.
Odin should marry someone that gives him further offensive potential and allows him to stick with swords/tomes. Peri gives cavalier, which can be handy for luna/armored blow; Camilla and Beruka give the wyvern line.
Niles' chief traits are his high speed and res. Strangely he has below average str for a physical unit, though in every run I've done, including my current one, his strength has ended up pretty high. His weaknesses are low HP, luck, and defense, so keep him away from direct physical damage and he'll be fine.
Niles' friendship and marriage options are clear as day: Arthur (the only one who gives him anything) and Mozu. Niles has better access to the bow initially than anyone else, and it'd be a little silly to take that away from him. By giving him access to Sniper, Mozu maximizes Niles' potential with the bow.
Mozu's highly useful in Nohr mode: she's one of the only merchants and the only sniper available. Her stats are good too: good strength and defense, great skill, speed and luck, and acceptable res and HP. Profiteer works well with her due to her high luck. Mozu can grab life and death, bowfaire, and spendthrift for a whopping +25 damage, allowing her to pretty much one shot everything.
Mozu's A+ options are Effie and Nyx. Effie makes her a solid tank, where she'll enjoy lots of Pavise and Luna activations; Nyx grants her Vengeance, Bowbreaker, Lifetaker, and Seal Magic. I tentatively think Effie's better. Mozu's stats are good enough to dodge many attacks and also not take much damage, making vengeance a little more questionable.
Mozu should marry Niles, giving him Sniper. She doesn't get much from him, unfortunately. Lucky Seven's ok.
Another lopsided unit, with excellent magic and good speed, but really bad luck, skill, HP, def, and mediocre resistance. Nyx is a glass cannon through, and a couple of gentle love taps from the enemy will completely dust her. Kill someone with her, then make sure she's safe by the end of the turn.
To shore up her weakness, Nyx can reclass to Outlaw early for more skill growth, and pick up Lucky Seven. Sorcerer and Bow Knight give some breaker skills, which are good; vengeance is a poor pairing because her HP pool is too small to make good use out of it. Seal magic from Dark Knight is a good way to soften magical enemies, though, and lifetaker can patch up any hits she's lucky enough to survive.
Nyx's A+ choice is obvious: Mozu gives Certain Blow and Air Superiority, both excellent for Nyx.
Keaton has incredible HP and Str, good Defense and speed, but very poor luck, skill, and resistance. He's a capable tank and a fearsome damage dealer if he gets the right skills. Unfortunately none of his alt classes give him anything too useful. Berserker's largely skippable since his end-game form should be a wolfseggner and there are much better rally units out there. Sol from hero won't trigger with any reliability due to his terrible skill, and while axebreaker might come in handy axe users aren't usually hard to hit anyway.
Keaton can A+ Arthur (which grants him nothing), Laslow (which gives him Bow Knight, which he doesn't really need), and Benny. Benny can give Armored Blow, which is nice. Also worth considering are Luna and Pavise, though both are skill activated and so won't be reliable by any means.
Keaton's massive HP potential means that he benefits considerably from Lifetaker, which he can get if he marries Nyx. Other interesting options are Mozu for Certain Blow, Azura for Darting Blow, Camaraderie and Warding Blow, and Camilla/Beruka for Malig Knight and Wyvern Lord.
Forget him. You have better units. He really has no place in Nohr mode outside of masochism.
Anna has good skill and magic, and great luck/resistance. She's got poor HP and defense, and ok strength and skill. Anna benefits greatly from a shining bow. She's a good money maker with her personal skill, profiteer, and salvage arms.