"I DIDN'T WANT TO DO THIS!!!", but today (or yesterday) is/was Olivia's B-Day. It's a. Nice day for a. White wedding.
Anyways, Olivia. As mentioned in Inigo's B-Day post a page back, she has terrible stat modifiers, as in the worst of any playable female in the game. As for classes, she has Dancer. That's it. Good day. What more do you need? Fine, I'll cover her reclass options.
As is she'll probably remain like your typical Dancer archetype with massive Speed and Luck, maybe Res, and not much else. Should you decide to reclass her, she's pretty well off with both the Myrmidon and Pegasus Knight class lines as she's ridiculously dodgy and her Strength will have a better opportunity to fix itself. As ultimately you'll want her to be a dancer, her skill preference is actually weighted towards Sword Master over either of the Peg Knight promotions since the Myrm's Avoid +10 skill is really good to bolster her high Speed and Luck (better for avoid than Luck +4, Speed +2, and All Stats +2 combined) and both Astra and Swordfaire do well in furthering her offense, far more than Lethality or Lancefaire can (especially since Dancers are sword locked in this game), and Galeforce would only be situationally useful to her. She's probably one of the first people to consider handing a Bride's Bouquet to since she might like Bonds to turn her into a sudo-Heron from FE9/10 (plus she can get some mileage out of Lancefaire and staff ranks outside of Falcon Knight).
Outside of this, her unique Dancer skills are crap to pass down since Luck +4 is the least useful of the whatever stat +boost skills in the game and Special Dance can only be used by her to begin with. That leaves Galeforce and unh.... Galeforce is really the only thing she should bother passing down to her son if she can help it. Although she might as well pass down Luck +4 to Lucina since there's really not much of a chance of her picking up another skill before she arrives (like not even Special Dance), plus its the only skill she has that Lucina can't get that isn't totally useless. She'll also appreciate the Peg Knight access and the least crappy physical modifiers of all her mother potentials outside of the Avatar (as in +0 Strength and -1 Defense rather than -1 Str and Def for Sully, -2 Str and Def for Sumia, and -3 Str and Def for Maribelle (for reference, Chrom only provides +1 Str and -1 Def)).
The tl;dr is that, she's the game's Dancer and she's one of Chrom's potential wives
for his shotgun wedding. What more do you want?