I’ve recently been reading 7 Seeds, which is Tamura Yumi’s other post-apocalyptic manga. The basic conceit is that five groups of seven people wake up in the future to find that most of humanity has been wiped out, and we follow each of the groups at various points, and watch as they come together.
Natsu, who is our first protagonist, is very shy.

I’m not as shy as Natsu is, but a lot of her thought patterns are very familiar to me. And Natsu feels true to me in a way a lot of supposedly shy characters don’t. For example, in Tamora Pierce’s Provost’s Dog books, we are told repeatedly that Beka is shy – but Pierce does a poor job of showing it. The only time Beka’s social anxiety actually seems to affect her is when it comes to public speaking. And being scared of public speaking is not the same thing. It surprised me, actually, because Pierce wrote Kel’s fear of heights so well in the Protector of the Small books. But I guess she doesn’t ‘get’ shyness in the same way.
Tamura clearly gets it.

Pretty sure I have had this exact conversation with myself.
Over the course of the series, Natsu learns to speak up, to do things she’s scared of, to relate to (and stand up to) her teammates. And this is just as important as is learning to survive in the future, in a hostile and unfamiliar environment.

Semimaru there is a character I’m very fond of, even though he starts off as a complete asshole. He gets a lot better over the course of the series, to the extent that it’s a shock to look back and see just how nasty he was. Now his relationship with Natsu is one of my favourite things. Because, yup, social anxiety can make you very self-centred, and Semimaru will call Natsu on that.

It’s not easy for her, but she learns. She’s able to become far more of the person she wants to be.
7 Seeds is unusual amongst post-apocalyptic stories in that it’s actually optimistic. To the point that you think, despite the many life-threatening situations, some of the characters are better off than they were in the present day. And Natsu is one of those.
In the present day, Natsu would have gone on as she was, often miserable, never able to connect. In the future, she has to connect to survive, and she’s a lot happier for it. She’ll never be an outgoing, vivacious person, but she’s able to stick up for herself when she needs to, and apologise when she needs to, and be there for people when they need her.

And then it turns out her two male companions do see her, and she has to rescue them as well. And one of them is judging her the whole time. (He has issues.) But she makes it anyway.
Natsu might not be the easiest character to like, but she’s certainly the one I appreciate the most. And I look forward to where Tamura takes her in the future.