Of course babying will take EXP away from other units, that's kinda the whole point - stronger units weaken enemies and the weaker ones take the kills. Unless you boss abuse, but that's really unsafe with underleveled units.

But yeah, you have to really put work into Fiona before she becomes usable. She starts underlevelled, her first usable chapter is an indoors mission which cripples her move advantage, and you don't really lose anything significant if you decide to bench her. Plus she's a spear user, and (almost always) needs a custom weapon so she can reliably finish anyone off.

You get at least three Paragon scrolls in the game, though - and Fiona has a good affinity and two free skills. So she's quite usable if you get her off the ground.

Fiona, Meg, Astrid, and Jill are by and large the hardest characters in the game to level up for me.

All come underleveled and have difficult times surviving.

Fiona and Meg are almost unusable without giving them BEXP from the start.  Astrid and Jill can get by without BEXP, but it's rough going.  Fiona, Meg, and Astrid all turn out mediocre so really the ROI of Fiona and Meg isn't high enough IMO.  Astrid is somewhat worthwhile and Paragon makes it easier.  Jill, while actually a bit harder to start than Astrid, has a much much better result.  She is simply stunning late game.

Out of all of them, Jill is probably the only one worth babying... but Astrid can come through too if you really want.

 The main problem with Jill is that she doesn't come with enough strength to hold axes so she can't double. Once she does get enough strength, she's ok. Calill also requires babying..(Missing Part 1 and 3 then coming for Part 4 with being in only one or two chapters in Part 2 IIRC) if you get the transfer stats though, she's a lot easier to come through with , especially if you have Largo's stats added as well.

You don't really have to baby Jill IMO. I gave her one of the Paragon scrolls and a custom axe - she ended the first part 3 Dawn Brigade chapter as a level 10 Dragonmaster (IIRC). She's frail, though - at least compared to Haar.

You should always keep at least one Paragon on the supply convoy so you can give it to anyone who needs it. Geoffrey doesn't really need his, and Astrid only does if you decide to use her.

If you want to use Meg, give her a custom sword (yeah, I'm big on custom weapons) with high damage output, and a Paragon if you can afford it.

Calil just kind of faded into the background like Tormod and his gang.  They were there but I didn't really expect to ever use them minus the chapters you first get them in.

I haven't done much Paragon shifting actually.  Didn't need to.  All the characters I wanted to get to their final tiers got there.

And yes, you really need to baby her.  It isn't long before she gets to the point where she dies very easily and (thanks to the STR problem mentioned above) can't kill effectively.  So you've got a long range character who dies easily and has a hard time killing.

Also, the colored parts really conflict with each other. 

As for Jill's "frailty," she goes to End Game instead of Haar because Haar is too frail.  ^_^

 I think Haar could tank better physically, but other than that it's a spot on statement. Haar can't double anyone endgame.

Although, I don't think it's that hard to make Jill get third tier. A paragon scroll pretty much fixes everyone's problem though.....

Haar had more attack and a point or two better defense but Jill had a full 10 points better resistance and equal/better everything else.  Haar would have gotten slaughtered by magic at the end...

I don't really consider Paragon use and weapon forging to be babying, but meh. She had the paragon scroll only for that single laguz chapter, anyway. :P

If you get transfer bonuses for your Jill and an energy drop, she can handle steel axes from the get-go. Without the energy drop, it takes a few levels. You'll have to be careful with her for, what, two or three chapters? She's already on her own after the first class change.

Then again, using stat boosters might be babying. Probably was one of the few boosters I ever used, since I almost always forget their existence or sell them. Oh well.

As for Haar's low resistance - have him sit on a wardwood tile and he can handle spirits (and the end bosses single target magic attacks) just fine. Purge users are a pain, but you don't really get hit more than once a turn from those.

Babying:

Weakening someone specifically to let someone else kill them.
Buying them special weapons you don't buy for everyone.
Giving them BEXP when you're not giving it to everyone.
Giving them stat boosters early in the game (versus at the end to cap stats).
Keeping them in the protective shadow of two or more characters.

That's what I consider babying anyway.