Um... I felt Teresa was at first just an overpowered shonen badass however I did feel she truly did evolve onto the perfect mother. Always doing what she felt was best for Clare in the end, be that risking being hunted by her own by murdering human bandits to save Clare to not killing Priscilla so Clare wouldn't be traumatized by Teresa murdering her (even with the risk she knew Priscilla was). It was a great character evolution... from perfect warrior to perfect warrior mother (and totally willing to give up the warrior part). I was moved by Teresa's opening up to Clare. At the end of her life Teresa was kinda like Ripley from Aliens doing everything to save the girl Newt, just with more crying and less nukes
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Don't get me wrong though... I did feel Teresa was severely overpowered and something would have to change at the end of Marked for Death arc for the story to stay any good. So as much as I loved her I was worried her power level was about to kill the story. Also, to be honest, I was more curious at the time of how the young happy Clare would turn into older traumatized Clare that we saw with Raki. So I knew something was coming but was too busy enjoying the anime (which I saw first before picking up the manga) that I really wasn't thinking too deeply about what would happen next but just enjoying the best anime I had seen in quite a while.
Your right with Teresa's extra chapter... to a large degree. Though it was also about showing us who she was before accepting Clare, however not much was added in that department other than she was purposely hiding her true power from the organization and lying to them while acting loyal but taking too much pleasure in scaring villages as retribution for people in her past selling her to the org. So most of the extra was just filler anyone would have assumed took place in her past and no other revelations and/or added depth added to her character.
In the end Teresa was a character with a purpose... just like Jean. She was designed to be someone so amazing that Clare would try forever to avenge her death, while Jean was meant to be so honorable that she would sacrifice herself for Clare one day.
Do I view either Teresa and/or Jean as amazing deep characters... not really (though Teresa did have more depth due to a back-story of her being sold to the org). Did I love them... absolutely.
What got me into the series was trying to figure out why Clare was so traumatized emotionally. However it was her back-story that kept me going after the holy city arc (though the end was good of that arc the beginning & middle sucked). So I totally agree with Teresa's death, even with her being so powerful, that got me into the manga. Cause when even the mightiest die you know the story will be interesting. I think what finally sealed the deal for me being a fan though was Clare and Irene talking about Teresa and how Clare living would be the only proof that she ever existed. That line said everything I needed to know about life and death in this story.
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Now, personally I want to see Priscilla not fighting for a bit and without the scent (no Raki and/or Clare). Why? Since I want more depth added to her character and I really really want a stronger emotional connection to her reasoning of why she wants to remember her past. I mean I get the logic of it... she just wants to have some happiness again but I want to feel her turmoil so as a reader I can connect to her drive to find the scent and restore her memories. Maybe a whole chapter of her just sniffing a scrap of Clare's flesh blow off in the battle (or a scrap of her uniform) and just feeling her happier days. Then as she's done sniffing she reverts back into the monster wanting another sniff of the scent.
Also I actually want to see Clare's scent effecting Priscilla's personality... since right now it appears Priscilla can smell it on Clare but that she's immune to its effects. Lets compare... Priscilla, the hungriest AB of all time, was able to starve herself for 7 years after meeting and smelling Raki however how has Clare, the source of the scent, effected her yet? I mean... come on! Major plot hole.
Right now I can understand Priscilla's drive but I don't feel it yet. Feeling pain is something Claymore usually excels at... so why is the main villain not included in this? We know she has a major reason for tracking down Clare, now but why does it feel so hallow? Right now she's just a machine chasing her target... however we know this isn't all she is since we know she did connect to some of her past self by hanging with Raki.