Avatar: The Avatar's varied modifiers can affect Severa in strange ways, but nonetheless she will still be an excellent unit. She will become a second Morgan, effectively giving her access to all female classes and the exclusive Tactician, letting her potentially serve as a taskmaster. Tactician gives her Veteran to speed up training while her promotion, Grandmaster, gives Ignis, a very powerful attacking skill and Rally Spectrum if making Severa a Rally unit. Since the Avatar himself can reclass into male exclusive classes, it is best to have her inherit something from those classes. There are no skills in particular that are recommended, but most chose between Counter or Axefaire due to their usability over the others. All other class inheritance and skills are listed below.
Frederick: Frederick gives Severa a great boost to her Strength, Skill, and Defense with only Magic and Resistance losing some points. Severa inherits the Knight, Cavalier, and Wyvern Rider class sets. Knight and Cavalier link up to each other, sharing the Great Knight class, giving Severa Luna as another possible attacking skill and Dual Guard+ to boost Dual Guard occurrences. General provides Pavise while Paladin gives Aegis, both of which are good defensive skills to shave off damage if she is hit, especially Aegis considering her drop to resistance. Wyvern Rider mainly provides two additional Weapon Breaker skills (Swordbreaker and Lancebreaker), from its promotions, both of which can be used in conjunction with Axebreaker, Bowbreaker, and Tomebreaker to make Severa nearly impossible to be hit by any weapon, but that is merely optional and can be used individually or in other combinations to suit the player's play style.
Virion: Virion gives an impressive +5 to both her Skill and Speed caps at the cost of only 1 point each to her luck and defenses and no bonus to her magic and resistance. Severa gains the Archer, Mage, and Wyvern Rider class. Archer can provide Hit Rate +20 in the Sniper promotion, but given her high skill, she should have very little trouble hitting enemy units unless playing on higher difficulties. Mage adds Sage to her promotions, but given her low cap boosts, Severa will not see much benefits from this class, nor its Tomefaire skill. Wyvern Rider again provides Severa the other two weapon breaker skills she does not already have.
Stahl: Stall gives Severa a well rounded boost to her stats with 4 points added to her Strength and Skill caps while her Speed and Defenses receive 3 points. Though she has a small drop to her magic, resistance, and luck, her other stat boosts will cover those. Severa inherits the Cavalier, Archer, and Myrmidon classes. Like before, Cavalier provides mainly Luna and Dual Guard+ from the Great Knight class and Aegis from the Paladin class to shave off more points. Archer and Sniper can be avoided unless Hit Rate +20 are needed since she already has Bow Knight through her Mercenary base. Her last class, Myrmidon, is a rare gem class, providing Vantage which allows Severa to attack first during an enemy turn if her health is down by half. Add this skill and have her attack or end her turn with a good attacking skill, namely Vengeance to capitalize her high activation rate of the skill and the fact that she needs to take damage to get the best results from it. This allows Severa to become a good reckless juggernaut character. Severa can also replace Vengance with Astra from her Swordmaster promotion since it allows her to attack 5 times and potentially critical on each attack, dealing a maximum of 7.5x damage. Swordfaire can be added if more sword strength is desired. Assassin provides Lethality and Pass, both of which are good situational skills, but are less desirable than the others.
Vaike: Vaike gives Severa the highest boost to her strength with a solid 5 points as well as a solid 4 point boost to her speed and skill caps. She suffers a small drop to her magic but that stat hardly affects any of her classes, but her resistance suffers a little. With her good speed and skill boost and growth rates, unless facing a Weapon Breaking knight, Severa should be able to dodge any incoming magic damage. Vaike, with his Barbarian and Fighter classes, can give Severa a male exclusive skill. Generally Counter and Axefaire are good choices, counter to return any sustained damage while Axefaire boosts her Axe damage. Severa also inherits the Knight class meaning that she'll have access to Luna and Dual Support+ through the Great Knight promotion while General will give her Pavise. Thief mostly provides a utility skill, Movement +1, while its promotions provide Lethality to instantly down enemies, though it will not activate often, and Pass, to escape from enemies if they try to corral her, through the Assassin promotion while the Trickster class mainly provides Acrobat to give her good mobility on all maps.
Kellam: Kellam's modifiers are similar to Frederick, being fairly solid with a lower Speed mod than others. Miracle from the Cleric class can be considered, but Severa will most likely not go into this class (or Sage) at all due to her low Magic and Luck modifier. War Cleric is a more viable option, and offers Renewal as another form of self-healing.
Donnel: Donnel's modifiers are much more evenly spread across all of her stats aside from a small drop to her magic and resistance. Donnel unfortunately provides the least amount of new class options given that she already can change into a Pegasus Knight and starts of as a Mercenary thanks to Cordelia. With Troubadour as her sole new class option, she has access to War Cleric's Renewal for its guaranteed health regen at the start of every player phase and Valkyrie's Dual Support+ to make her a better Pair Up support unit. Thankfully Donnel does have the Fighter class as well as his own exclusive class, the Villager class, to give Severa's skills to inherit. Fighter basically only provides Counter while the Villager has Aptitude which boosts her growth rates by 20% and Underdog which boosts her hit and dodge rate as long as her enemy is a higher level than her. Which ever skill the player chooses for her to inherit is up to the player, however keep in mind that Aptitude is only useful when training her and loses its usability once she caps all of her stats.
Lon'qu: Lon'qu gives Severa the largest boost to her Speed and Skill stats with an impressive +6 to her skill and speed, while both of her defensive stats receive a small drop to their stats. Overall, this Severa has the easiest time activating skills and has an impressive dodge rating, meaning that her defensive drops are a little less hurtful when she is hard to hit. As mentioned above, Severa gains Vantage, Movement +1, Astra, Swordfaire, Lethality, Pass, and Acrobat from the Myrmidon and Thief class and its promotions since they are linked. Wyvern Rider gives her the other two weapon breaker skills, giving her all five. Using these makes her an amazing defensive unit, especially with the Swordmaster reclass. Lon'qu is one of the best fathers for Severa given that she receive a plethora of skills to make her a very diverse offensive unit.
Ricken: Ricken gives Severa balanced mods, with more of a focus on her Magic stat than her Strength stat. As stated above, Severa won't be as good of a Sage or Sorcerer than a naturally magical unit like Laurent, though she may be able to make a decent Dark Knight or Falcon Knight with her modifiers.
Gaius: Excluding the Wyvern Rider classes, Gaius provides the same classes as Lon'qu, and even similar modifiers, with slightly better Strength and no negative Defense or Resistance mods. Severa's inherited classes work the same as Lon'qu so consult the above to see how they will work for her.
Gregor: Gregor can give both Swordfaire and Axefaire (provided that Severa inherits the latter skill from him), and is the only non-Avatar father to be able to. With both skills, Severa will make a great Hero, though given her mods, she can also make a great Swordmaster or Assassin. She can grab Astra and Lethality from both classes as better attacking options over Vengeance, though having Vengeance and Vantage in her skill set can be helpful in a tight situation should she ever be faced with one.