Kaze:
Already been discussed, but Kaze benefits tremendously from being the lone starting Ninja of Conquest, and even then, certainly makes for a better one than Laslow would. Ludicrous resistance, ludicrous speed, nearly unparalleled availability and utility on top of everything else with Locktouch. Later, you're spoiled for choice because both of his promotion options are amazing (though I'm inclined to say Mechanist is better for him). His only downside is that you don't naturally get a weapon that's maximum overdrive for him like Felicia does, but that's still totally fine when he's doubling so much and Poison Strike exists anyways. And hey, Lethality is funny when it actually works when your opponent wasn't about to die from a hit anyways (just like crits, rip).
Selena:
First up, Selena starts the game with very solid growths in most areas. She can double most anything from the get go, and her speed snowballs so much that even the fastest enemies in Conquest are liable to get doubled by her, never mind without the help of tonics or support partners. She has good bulk and good defense, and very good growths for both. Combined with her speed, she's immediately practical to send out, which is good because she arrives at Chapter 10, land of experience spikes.
Yeah, she has low strength bases and mediocre growth in that department, but here's my secret to fixing that. Just pair her up with Arthur, whom is a big bundle of strength and skill. Keaton is another good pick for her. Or you could pair her up with Wyvern Lord Camilla. Beruka fits with her well, too. Peri does in a pinch. If you absolutely need to for whatever reason, Xander also repairs her strength. Selena is spoiled for supports that fix her weakpoint. I don't think skill is that big of a deal since you're usually going to get 'enough' unless you're really unlucky. She doesn't need anything else, maybe some tonics if you really wanna push her limits because she'll just slap so many people down.
I think the beauty of Selena is honestly that it's basically impossible for her not to suit the needs of your army in some way, too. No one fucks up by sending her into Pegasus Knight, though she does need to deal with E-hell, which isn't that bad that early, and as you said, the combination of flight, stave access, and great skills only she can use (or gift) make taking that route incredible for utility. But if you don't feel like you need that, she has two other amazing classes open to her.
Hero locks down her survivability with the bump in her HP growth and early access to Sol, which is of course an incredibly strong skill for keeping a character sustained, and again, Selena is tough enough and fast enough that she's not living off her proc rate, ignoring Guard Stance body blocks on top of all that. It's, IMO, her best class, since it compensates for Selena's weaker areas and Axebreaker down the line is still quite good for later. But Bow Knight cranks up her movement and gives her a decent ranged option that pairs nicely with her existing stat spread, on top of the goodness of Shurikenbreaker. It's all good stuff.
I think her only downside is that her Personal is really neat and flavourful, but it's also essentially useless.
in short:
selena owns, and her only real flaw is that laslow is so much harder to justify with her around