I've talked a little bit about it Isabelle was a cis boy before here (https://neospring.org/+r/f3ed44f87b) but I'll happily talk about it more! Both cis and trans this time.

I ultimately think that Isabelle's "boy personality" already exists when her genderfluidity shifts boywards. The traits of his that become more prominent then—competitiveness, protectiveness, violence, a desire for control—are born of having Alec for a male role model. If Isabelle was a boy all the time, whether cis or trans, I think he would be ashamed of his submissive desires the way Isabelle is ashamed of her dominant desires by default.

However, he would definitely still have them. To reconcile the desires he would be bratty and manipulative with his male doms, but wouldn't really enjoy it as much as he would total submission, which he would find with Cy (assuming Cy is down to dominate a man other than Eden).

In general boy!Isabelle is more in touch with the anger and aggression that our usual nb!Isabelle already possess, and the ideals he tries to live up to are different. Basically every trait of Isabelle's that is societally seen as "masculine" he leans into, while leaning away from things seen as feminine, but the fundamental personality is the same.