Read on Twitter
everything we know about yaz because she does have character development and i will die on this hill: a thread 
when we first meet Yaz, she’s a probation police officer who wants to do more and believes she’s capable of it
we find out that her and ryan actually went to school together which is pretty cool
she clearly has a really close relationship with her family: her sister (sonya), her mum (najia) and her dad (hakim) and we later find out in demons of the punjab more about her relationship with her nanni (umbreen)
in rosa parks, we start to see the arc of yazs struggles - she really wants to make the world a better place, she was also in the rosa parks class at school
in demons, not only do we get to see she has an extraordinarily close bond with her family, but that she’s a fiercely trusting and loyal person who feels a sense of betrayal at umbreen, but that she’s also extremely empathetic and forgiving, and holds her family close
the witchfinders is when we really see her start to open up about her struggles at school. she mentions izzy flint (the cow) and we see how deeply this has affected her and her life choices
yaz is also fiercely brave and intelligent, some examples in s11 include in ITYA and TBORAK, where she has knowledge that no previous companion appears to have been able to grasp
however, we all know that s11 was really for graham and ryan’s arc and it’s s12 where yaz really comes into her own, right from the get go
yaz is clearly very eager to travel with the doctor and it’s almost bordering on recklessness as she’s willing to risk her job, and even her life to do so, she also tries to hide her fear and her pain, as she attempts to quickly move on from the kasaavin realm -
probably in an attempt to not only prove to the doctor that she’s able to still travel with her, but to distract herself. it’s also strongly hinted throughout that yaz has a problem with her self worth, which ties in with her penchant for reckless behaviour
this manifests in a few ways. the look of pain in her eyes in NTNOT when she’s called not important, and the immediate spark of confidence when the doctor disagrees. however, she also throws herself into danger, which appears to be to prove herself to the doctor
and show that she deserves to travel with her, a couple of examples of this include in praxeus, where she’s desperate to prove that she can do things without the doctor but also in the timeless children and HOVD where she tries to throw herself into dangerous situations
yaz clearly has this strong need to prove herself even though she’s evidently intelligent and likeable from the get go, but this comes to a head in yazs mental health storyline in CYHM
CYHM strongly hints at a dark past for yaz full of bad decisions that might have been made had her sister not intervened (again showing her tight relationship with her family) and we see her motivations for becoming a police officer solidified
and although these have been strongly hinted throughout its nice to have confirmation that yaz genuinely wants to help people and make a difference, something she also thinks travelling with the doctor can help with
yazs worst flaw is that she clearly has a constant need to prove herself. which isn’t helped by the fact that 13s doctor is the most distant, and so yaz continuously tries to win her approval
yaz is also the most deeply affected by travelling with the doctor. she seems shocked at the end of CYHM when ryan even suggests that they could stop travelling in the tardis, again hinting that yaz is fiercely adventurous but also reckless
at the end of the timeless children and in HOVD we really get to see her feelings for 13 physically manifest - she can’t imagine her life without the doctor anymore
in ROTD we see yaz has abandoned her job and her old life - she’s entirely obsessed and focused on the doctor, she doesn’t know how to process it and this one thing that’s given her validation and made her realise her own worth has been stripped away from her and she’s lost
in conclusion, yaz has character development but not all character development needs to come from saving the planet or being the most important woman in the universe. yaz has grown so much, but also we still see her struggle with her confidence and self worth.
she’s brave but she’s also reckless. and she always puts herself last because her self worth has been torn apart from years of bullying and mental health struggles. and the doctor has been a way to combat this but now she’s left on her own again and she’s struggling.
a lot of people can see themselves in yaz. myself included. she’s someone who has struggled and who can’t quite believe that she has worth and so feels the need to constantly prove herself. just because you don’t like a character, doesn’t mean they aren’t important to someone.
so please think before you start saying she has no character development. and if you read all this you’re a real one and feel free to add anything to this x
also, just want to add, that yaz places a lot of value on life (though her recklessness can outweigh her own self survival instincts) - but she has a lot of empathy for others, most notably in kerblam and demons, where deaths hit her hard. she feels a lot of others pain.
also her own confidence grows a lot when she’s travelling with the doctor, if you compare her now to when we first saw her in TWWFTE, she’s a lot more trusting of her own decisions and the doctor has played a part in increasing her trust in herself and her instincts
but, again, with all the happiness and self determination she’s gained from her travels, she has also become somewhat dependent on the doctor to have this self worth and when she’s not there she’s desperate to get back to this person she was with the doctor, and is reckless
and finally, i get that some people feel her development isn’t complete - and that’s cause it’s not! there’s a reason she’s staying on for s13, yaz’s story arc isn’t complete and that’s okay, but there has definitely been a natural progression for her and her character
