Céline was a missed opportunity to make a good hybrid unit. Let me explain!
1) Why hybrids don't work in practice: I feel like fire emblem is afraid of making good hybrid units, in a way I get it. Having someone with both good offensive growths would be better than fielding two specialized units, but there are plenty of ways to balance it (a glass cannon assassin like character, with both strenght and magic but with Nyx level of bulk, that relies on dodging and can only take one hit, both physical or magical) but instead they go the "jack of all trades route".This, funnily enough, hybrid classes rewards specialized units more. A caster with high magic but low speed and bulk will prefer mage/dark knight to patch up their weaknesses, meanwhile a jack of all trades will need sage to make their magic competitive. The only "advantage" of mixed units is their versatility, allowing them to choose a physical or magical class, but they tend to be the worst mixed units statistically speaking, barely able to reach solid numbers in both (meanwhile a mage might use a lance or sword against an high res wall and deal decent damage while maintaining their specialization in one main stat, better to have 30 magic and 18 strength than 24 in both).
IS also avoids giving units both high offensive stat bases and growths (with some exception). But even if they were, I feel like most units would opt for the mage route if they had the chance, enemies on average have less res than def, so if a character were to have 45% growths in both strength and magic, instead of getting 55% in both with a mixed class, they'd rather go mage for the 70% instead. Or in the rare case they opt for a physical class, they choose the ones with good magical weapons (shining bow Nyx/Elise Adventurer Nina, Revelation Priestess Sakura, bow knight Engage Anna, Levin Sword Griffon Pandreo etc...)
The best "true hybrids" have always been units with higher strength than magic or with an hard time getting access to tomes. Think of Adventurer Niles with both physical and magical bows, Flame Shuriken/Felicia's Plate + Other daggers or shurikens for Master Ninja/Mechanist Saizo, Cupido Fogado, even malig knight Camilla, while statistically inferior to wyvern lord, has some options with bolt axe despite her strength being like 10 points higher than her magic.
2) Why Celine doesn't work
Now with all the issues I pointed out I want to talk about Céline. The thing about having an exclusive class is that... You don't have to worry about designing an unit, you can just tailor a class to make them into one. Ivy's growths are shit, her magic is ass, but her class saves her with +25% growths in magic, bulk, decent build, a flying mount and good base stats (10 base magic when promoted), so her 30 becomes 55%, Anna, Citrinne, Gregory or Pandreo could reach higher numbers with Sage 30% but that means having less movement, bulk, speed and constitution, alternatively with mage knight they can get 25% (putting them at 75%), but the mage knight has a base magic of 7, 3 more points when you, at best, have 20 levels to grow, makes the 15% higher growths less impressive), and they don't get staves and her high movement (access to levin is good but if you have the dlc Zelestia can do the same as a sidegrade Ivy), and Ivy gets Nova too like sages.
The issues with Céline: Now let's look at Celine. Her growths are impressive, but like other hybrids before her she decided to invest in... Luck, one of the worst stats along with skill (unless you're Arthur, Knoll or Ivy you just need enough luck to avoid crits, and unless you're Elise, Nyx or Diamant, enough skill to hit). Like Odin before her, she's painfully average, and their highest stat is strength, needing an exclusive class to patch their shaky magic. And if course her Ignis is a a percentage skill based on... Dex? She's got like 29, she's no 22 diamant but close, Timerra and Alcryst can get almost coin flip chances to trigger their specials, while relying on percentage base skills isn't usually consistent, with brave weapons you can almost guarantee one proc (it's gimmicky but it's something, she doesn't even have that). Her luck isn't useless, with divine pulse she can do something similar to Hortensia, and she gets staves for utility after her mediocre combat loses its sparkles, but engage gives you so many staffers (ivy, hortensia, a griffin, mauvier is almost always being deployed as you get an additional deployment slot and I doubt you'll have someone else trained as he has good stats and comes at level 12 if I'm not mistaken), there's also Micaiah, and while martial master isn't good as an offensive unit besides Eirika lunar brace quad hits, you'll probably have one for chain guard and/or bonded shield.
Another issue with Celine is swords, lances, axes or daggers are better weapons for hybrids due to their 1-2 range variants, of course swords are better for levin access, but that means having less options for enemy phase, she'll rarely use her strength besides emblem attacks as a result of that. If I was to design
3) Céline as a true hybrid:
Let's make her good, let's assume she's get knives, spears or hammers instead of blades. I'll use Vidame (noble will have the same stats with like -5% here and there like how it already is in engage).
5% HP, Str: 25% (+15), mag: 20% (+5), skill: 5%, Spd: 10% (+5), def: 5%, res: 10% (-5), lck: 5% (-15%), bld: 0
HP:21, str: 7(+1), mag: 10 (+1), skl: 6, spd: 9, def: 4, res: 5 (-1), lck: 3 (-2), bld: 6 (+1).
Base Growths. 50% HP, Str: 35%, mag: 35% (+10), skill: 35% (+5), Spd: 45%, def: 35% (+5), res: 35% (-5), lck: 35% (-15%), bld: 5%.
She'll reach 60% strength growths and 55% magic growths, but she's stuck in this class to work. I feel like this solution is so dumb it works. Because it's based on old fire emblem games, without reclassing you could give units high growths in bad classes or bad growths in good classes to balance things, meanwhile Engage took this option. Now you can use Celine with good growths but she's stuck in her exclusive class to work, otherwise... She's basically Jade.