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.
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. 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.
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.
Gunter: Forget him. You have better units. He really has no place in Nohr outside of masochism.