Lets look at it this way.
Haar is Haar for 2-P, Leanne is Haar and Elincia (just a wild guess). That's cool.
Leanne is two units who have a tough time with enemy wyverns with high defense, and don't even do much damage to start. Until Haar comes in. Then she's Haar and another unit.
Then, Haar exists for the entirety of the GMs in Part III. Reyson can at least negate the availability gap with four units, as well as having somewhat stellar availability; Leanne is available for only one chapter and does well in that chapter (minus the stone throwers that kill her in two shots) whereas Haar is doing quite well for every other chapter. She does better in Part IV presuming you put Haar with the Silver Army, but she also has a Purge tome to fear in 4-3 (they both do, but I'd reckon they're more likely to snipe the one they can kill in one round) whereas Haar can take like one attack from it and still carry on fine.
Final Chapter, Leanne > Haar. But guess what? Haar's beating her in availability and his stellar use during the amount of chapters she has over her, and they all pretty much bear equal weight (although I'd argue the Part IV chapters hold a bit more weight - not enough to keep Leanne above Haar).