.... Elrena in no way gave the impression she was some trickster or vulgar though? I thought it's pretty clear by now that nobodies don't always match their somebodies and can change distractedly? Isa turned from the nagging friend to the murderous jealous ex-friend for example.
The core of Elrena seems to still be there in Larxene (teaming up with Marluxia no matter what), but clearly whatever she went through has changed her. Same with Marluxia, Mr. I'm a sweet big brother to using a child to turn another child into his puppet.
And of course, the biggest example is that Roxas' personality is pretty different from Sora's, despite being Sora's Nobody. Sure, memory shenanigans, but still.
There's also a huge time gap that we don't know about that could explain Elrena's personality change as well. We don't know when she landed or even if she made it there without any incidents, because we also don't know how Lauriam and Elrena became Nobodies. For all we know, the lifeboats those two were using were even more broken than we thought, and it ended up turning them into Heartless. Or, when they eventually made it to the real world, maybe Darkness played a role in what happened.
It's also very likely that Marluxia and Larxene never had those memories as Nobodies, so the personalities they have in that form are a direct result of them starting from a "clean" slate and whatever influence they've experienced post-Nobodification. When Sora defeats Marluxia in KH3, he starts to get his memories back, so for whatever reason, he didn't remember his time as Lauriam until he's about to be recompleted. As for why there's memory issues, I don't have an idea yet since Saix remembers his time as Isa (and I'm sure Axel remembered his time as Lea), as well as a majority of the Organization members (or at the very least Vexen and Demyx). So something probably happened once the gang reached the real world, and that might be why the changes are there.
I will say though regarding Lauriam, even though he's basically everyone's Big Bro, let's not forget he tried to ice Ven twice. The second time was while the boy was sleeping, on top of that. Considering the circumstances, I understand why he feels the way he does, but there is
some precedence there that shows how he could've ended up being the Marluxia we know. UX does a great job of how violent he can be once he loses his cool.