Ok, sorry it's a bit delayed, but here's some of my Jack x Ace headcanons. I decided to review Jack's character before I started on these. As a reminder, I mostly follow the Japanese version of the game, and I'm still in chapter six. Please keep that in mind for my characterizations.
(The headcanons will be in no particular order)
Bit of a time skip, but I see them as a power couple as housewardens
Jack is 100% savanaclaw's housewarden, but I actually see Ace being the vice housewarden for heartslabyul
Other students tend to go to one of them when they need help, since Jack is extremely serious in his position, and Ace can give them great advice.
They're also great to talk to since they both have emotional intelligence
I believe in Jack being secretly down bad for Ace. He'll rarely show it due to his tsundere tendencies, but he's a total simp.
That's not to say Ace isn't, but Ace will eventually admit to it (Jack never will)
Jack starts to wait for Ace in the courtyard so they can walk to class together. He eventually has to start going to heartslabyul to make sure Ace actually wakes up on time.
Jack helps Ace study since he clearly has the capability to do great academically, but Ace struggles with putting in consistent effort
Jack helps Ace with his issues with authority, and stops people from constantly comparing Ace to others
After they get together, Jack 100% starts to slowly call the upperclassmen out on comparing Ace to Deuce
Ace fell first, since Jack was in heavy denial
Jack had to be the one to confess tho, since Ace kept trying to hide it, and told himself that they wouldn't work out
After first year, I headcanon that Ace starts to put a bit more effort into his appearance, so by third year he's a certified pretty boy
This means that after first year, more people start to crush on Ace, and Jack occasionally gets jealous
I low-key struggles to come up with headcanons even tho I love this ship and gave it as an option in the poll. But lemme know what y'all think, and if I should do more.