Want to know who your favorite characters/characters you don't really understand in WST really are? Comment here and I'll give a detailed description of their personalities at their core. It's so much fun for me omg!

Alexander

Alexander will sacrifice everything he has - every piece of him - for the people he loves. He would go to the ends of the earth for them no matter what that costs him - but even so, if it's between one of his loved ones who has done something very wrong to hurt the world and the good of the world, he'd choose the world first, even though he'd be very broken up over it. For him, the good of the world has to come first because he knows he's not the only person who loves people as much as he loves his friends/family and they don't deserve to die because of something someone he loves did. He loves really deeply and is very understanding of the world. Very empathetic. Those feelings make him fragile and so to be strong he must build himself up and train hard so he can be good enough to defend/protect his loved ones so he can be strong (mentally) too  (aka his family and friends give him the need to defend which gives him strength). What people don't realize is that yes, they all rely on Alexander a lot, but that Alexander relies on them just as much. Alexander is broken and haunted by past mistakes and guilt but he doesn't let that get in the way of keeping his friends and family safe, but when the battles are won and there's no one to protect but himself, something small eventually (inevitably) happens and he just breaks, but Rosemary is the glue that can hold him together and that's part of why he loves her so so much. To him, she is peace and love and embodies everything he wants to protect (and low key partially because she looks cute in a mini skirt, but that is besides the point)  and so he's just enchanted by her very existence and her very being and when she touches him he goes back to a time when he wasn't so broken and he wasn't so scared and he wasn't so guilty and she makes him want to be the honest man he could be if he just could take down Braylon so he's trying to work hard to do that every chance he gets. He also believes in second chances partially because of just who he is and partially because he's banking on it to take down Braylon and to be an honest man who doesn't hide things from Rosemary and who doesn't enlist a young teenage boy who's so scared out of his mind to heal Alexander's broken mind

Henry

Henry has been the apple of his mother's eye since he was a baby. He was taken from social event to social event in everything from strollers to baby harnesses to tiny dress suits, but that meant that he didn't have much time with his siblings. From a very young age he was separated from Violetta and Alexander, which really affects him now because he isn't close with either of them and he constantly feels left out. By the time Marilynn mostly stopped taking Henry to different events and gatherings and he was free to hang out at home, Alexander was in the silver towers. Violetta was still at home, but she didn't really have an interest in hanging out with her little brother because they just didn't have that connection and would rather hang out with Claire. However, Violetta did take Henry over to the Dizznees where Henry could hang out with Jasper, someone who wasn't too busy to see him/actually wanted to see him. That's kind of why Henry is so protective of Jasper and does his best to shut Amy out. Jasper was the first person who had time for Henry AND wanted to see Henry because he thought Henry was a cool person. However, Henry is also kind of jealous of Jasper because Jasper is close with Alexander, HENRY'S older brother. Aha yes we are back to Alexander's part in Henry's story.
Like I mentioned before, Alexander wasn't really ever able to be there for Henry because Marilynn took Henry everywhere. However, as mentioned in like 3.8, Alexander was able to watch over Jasper and take care of him like he would've taken care of Henry. That ended up in Jasper absolutely adoring Alexander and Henry partially loathing Alexander. Henry was too young to know that Marilynn was (unintentionally) the cause for the separation of Henry and Alexander and thinks that Alexander abandoned him for the rest of the world (particularly Jasper). Henry doesn't know how much Alexander wanted/wants to have a connection with him and know him but the timing just was never right to be able to make that happen and now Henry won't give Alexander the time of day for that. Henry blatantly ignores/pushes Alexander away which really hurts Alexander, and favors Celeste because she isn't "mushy-gushy" like Rosemary (who he thinks is totally lame because she's gentle and he thinks that's a sign of weakness due to being teased as a kid by kids at Marilynn's social gatherings for being a momma's boy) and she definitely isn't Alexander. Celeste tells him like it is and shows him robots which is cool, even though she sometimes gives him advice. Because Celeste is an older sister figure who he lets give him the time of day, Henry isn't TOTALLY jealous of Jasper and Alexander's relationship and can put Jasper's title of being the first person to want to see Henry because of his personality over the fact that  Jasper "betrays" him by being closer to his older brother than he is.
What also helps this is that Henry blames Alexander for Marilynn being attacked so brutally a couple years ago. Henry thinks that if Alexander was fast enough and strong enough, Marilynn would still be able to walk and such. Henry, when pushed a little too much by Alexander, who longs to be close with Henry, likes to bring up his feelings in this when trying to scare off Alexander, but always is confused as to why Alexander turns a sickly shade of pale and almost hobbles as he leaves. It makes Henry feel a little guilty, but not guilty enough to remove it from his arsenal... which is always a very affective way to end a conversation with Alexander, whose mind usually breaks after Henry pulls THAT card... but Henry doesn't know that.

Poppy

Poppy is two so she doesn't really have a big dramatic story like Alexander and Henry do, but she has the pure-little kid honesty that I don't think any other character in WST-KOTLC has. She'll tell you exactly how she feels and asks exactly the question she's thinking because she's not scared of consequences or anything like that. She gives her heart freely to others (see 4.6) because everyone deserves a little love, no matter what they've done, right?
She loves her parents a lot and sees Alexander and Rosemary as another set of parents tbh. All of her older siblings and cousins are her favorite playmates because they're just so gosh darn cool. Yeah, Poppy goes to daycare with Rosemary a few days a week so she can hang out with kids her own age, but they all treat her differently because their parents tell them that she's a Dizznee and she doesn't like that - I mean, she likes being a Dizznee/the teacher's little sister, but she doesn't like how the other kids want to play with her because of that instead of for the fact that she's a cool, funky, creative kid. Poppy's family members and close family friends are genuine with her and even at a young age she knows that's what she values. She'd rather have a few real friends who like her for her than a bunch of friends who want to be her friend because of her last name.
Don't worry though, she isn't sad, she's having a blast and when she gets older as can understand/verbalize more things, she'll successfully weed out friends who'll stick by her side no matter what :)

Rosemary

Rosemary is the oldest of all of her siblings (being 5 minutes older than Celeste), and is often seen as the most fragile of her siblings all because she doesn't like fighting. She doesn't like people getting hurt and will do her best to prevent it, that is true (it makes her sick), but if innocent people (especially children)/her loved ones are getting hurt, she will NOT stand for it. Consider her the Bruce Banner of family love and justice. If something happens to an innocent/loved one, Rosemary will suit up and take down whoever hurt that person/those people without hesitation (but with mercy, because the criminals are people too). She can rise above her fear and do what's needed to be done without being haunted by her choices afterwords because she knows she did the right thing, even if that right thing was hard. The stories that people told her (parents/aunts/uncles/grandparents/teachers/friends/etc.) of the war with the neverseen when she was younger scarred her as a child because "how could people be so awful? How could they hurt people like that?" And so she's pretty scared of her loved ones dying. She's most scared for Alexander because of his job at the CFR and got extra healing certifications (like human Red Cross) soon after he got the job, which was one of the only ways she felt comfortable with Alexander constantly being put in danger because of his job. The other way was she convinced Alexander to put her as his emergency contact (which wasn't very hard to do), so whenever he gets seriously hurt, the CFR calls her and she brings something for Alexander and sits by his side until he's well enough to go home, and then she takes care of him there.
To branch further off of Rosemary's love of taking care of people, Rosemary is essentially a second mom to all of the Dizznee and Vacker kids. It all started when Theo was born and little Rosemary was so excited to have a little brother that she insisted that Biana and Dex let her help take care of him and so Biana and Dex were like "sure okay", not thinking she'd actually go through with it for as long as she does and she learns the ropes really quickly and her skills are always complimented by other adults and Celeste is her right hand man, even though Celeste doesn't particularly like kids. After Theo, Rosemary kind of took all the Dizznee/Vacker kids under her wing because she saw them so often and sometimes the adults just wanted to hang out by themselves yk? Rosemary, in this motherly sense, is especially close with Poppy, Jasper, Amy, Claire, and Isaac. Rosemary is Poppy's prime baby sitter (but when Rosemary is busy, Poppy is taken to Alexander instead since Celeste is always working with her tools and such which is kinda dangerous) and her daycare instructor so naturally they spend a lot of time together. Jasper is close with Alexander and ships him with Rosemary, so he kind of sees Rosemary as Alexander's wife which is really cool to him and he loves being sandwiched between the two of them because he just feels really loved and Rosemary always takes care of him when no one else is looking. Rosemary can relate it Amy and Claire on make up and clothes and girl talk because Rosemary was where they were when she was there she and she's very understanding of different perspectives (similar to Alexander) and so they go talk to her for older sister advice on boys and clothes and friendships and such. Rosemary is close with Isaac because she has always looked out for him and cheered him on and she (as an older sister figure that he doesn't have by blood) just makes him feel special and taken care of by a cool older sibling tbh, there's not much else to it.

Jasper

Jasper is tired of his friends fighting. Oh my gosh. It's awful. Because of his friends always fighting, Jasper uses humor to cover up his distress as he doesn't really know how to make more friends (since he's been around them all his life) and he's struggling to piece together what friendships really look like. He, like Poppy, likes to make people happy, but he also doesn't know how to deal with arguing and such so he kind of just slips away and let the people fighting figure it out themselves. He's very curious and likes to ask questions and loves feeling safe when he's with the first waves. He's got good instincts of character and is a loyal friend. Jasper is still learning and growing and that's what he's doing in WST/my mind rn so there really isn't much to say on him at the moment because he'll grow from here on the page in my mind and his growth is the kind of thing that needs to happen in time as I'm writing.