I like Dark Magic users by default, sooooo... yeah.
Plus in terms of just an in-game play-through and stuff she is the best tome wielding, assuming no class change and crap like that, that isn't Robin. Miriel is ok and can get the job done but will generall be outclassed by Tharja because of Tharja having better Spd growth, and access to Dark Magic. Ricken is lol worthy, like really, why does a Mage have a 35 growth for Mag? Henry is kinda eh to with his 35 Mag growth but at least has has decent speed growth and he is pretty bulky for a magic user, even for a Dark Mage when compared to Tharja. Once you Lissa and/or Maribel promoted you get 2 other decent magic users, they still don't have the most desirable stat growths but considering how they start out a Stave users they are able to level up easily and fast compared to other characters (really though, why can I level up Stave users in Awakening and Shadow Dragon super easily and fast, like I can get them to max level in just a couple of chapters but stave users are such a pain to level up in PoR/RD and SS, like I can get 4 characters in the Dawn Brigade chapters to promotion before Endgame (not including Micaiah who I can also get to max level easily before end game) but I'm lucky to get Laura to level 10) which makes them prime Master Seal candidates rather early which generally balances out their somewhat iffy growths.
I have no idea where you got your sources, but I think you were just RNG screwed. I don't mind if you say that Tharja is the best mage in awakening, everyone can have different opinions, but don't lie about growths. On the internet, everyone can check your "Facts".
Miriel has a 60% speed growth as a mage, exactly the same as Tharjas. This means that by the time Miriel reaches Tharja's level she will probably be faster. Miriel has seven speed at level one. Let's not forget that she also happens to come far earlier(mission 2 in comparison to mission 9) so will probably be at a much higher level then Tharja if you use her even semi-regularly. Hell, Miriel is actually considered to be one of the BEST units when it comes to Lunatic because she's fast enough to avoid being doubled regularly (and one hit killed after an hp growth or two) and able to kill all of the knights that come with the Feroxi Chapters. This means purely from an availability standpoint, Miriel can become your first lead mage, capable of destroying everything that comes at her.
I personally hate Ricken. I think he's a pain to deal with on his chapter because of how squishy he is on join time as well as his inability to double, but his speed growth and magic growth are nothing to sneeze at. He does not have a 35% magic growth. Its 55%. And his speed growth is 50%. Neither of those are that bad.
Tharja beat both Miriel and Ricken in defense by 10% growthwise but Henry beats Tharja's defense growth by 5%. Henry also has a far stronger strength growth allowing him to be the best Dark Knight,(on top of a 50% magic growth) as well as pass his awesome classpool onto physical units without gimping the children characters.
Come to think of it, Thar'ja is pretty bad when it comes to a no grind reclassing run, as her class pool includes
Knight, and Archer. Two classes which need a
Strength Growth and a Strength Base.
Ricken's class pool is also pretty bad if you aren't planning to grind. Cavalier and Archer are terrible unless you statboost like crazy. Sure you get Luna, but at what cost?
Henry's class pool are both physical units, but he has a 40% Strength growth. His skills also combine pretty dang well into a critical hit build, once you take a step back and see that he can stack gamble, Wrath, on top of his already high skill.
And Miriel man, she's got the best Classpool over all these mages as she's packing Dark Mage and Troubadour.
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Now as a Character. I personally found it creepy that a black mage would follow someone all day, count the number of times they roll in their sleep and whatnot. Maybe you're into that stuff, I don't care. What I do care about is how Tharja uses Noire as a guinea pig. She tests out her hexes on her own daughter. Gives her runny noses, bad colds, curses her all the time; there is a reason why Noire turned out the way she is, and there is without a doubt in my mind that Thar'ja played a role in it. She has nice supports, she's has a nice heart deep down, and is shown to be willing to die for her daughter as shown in the future past but that doesn't change the fact that she makes her husband cry to sleep at night.
I mean that's the major reason why people have a problem with Tharja. She's a decent character, but she does a lot of bad things.