Has A.B.A been training or practicing with Flament Nagal? Because in Strive, it looks like she knows what she is doing compared how she was in the old games.
In the old games, Nagal seemed to be doing all the work but now it looks like a reverse unless Flament Nagal is becoming more reluctant to fight these days.
At first (if you'd been following the Drama CD script) it was revealed that at the very outset A.B.A. didn't even know Flament Nagel* was a weapon!
She just thought he was a giant Key she happened to like and find attractive!
He dropped several hints after that during her duel with Slayer, and agreed to work as her partner afterwards.
Time has passed and Slayer DID state he would check in on them in about 10 to 20 years, so hopefully that will happen soon~
Some minor differences as well is:
In her original Isuka version: Evidence Concealment was a giant golden Key poke, but it later became an Alchemic chain reaction... this later became the "Keeper of the Key" where the Gate of Truth opens up and the Darkness attacks.
The Union command Grab: instead of being shocked at his transformation, she asked the question "What's your Blood Type?"
(Ketsueki nani ka ta?), this was directed at the opponent, since she was taking their blood from them (this is also an indirect reference to Fanny's old command grab from Guilty Gear Petit).
She'd also feed Flament blood generously with her Blood packs.
The change in focus to Jealousy (and Love) makes sense, since their relationship has progressed a great deal since they started out, but it also reveals more about the nature of Flament Nagel as a Foci (Magic Focus Beast).
Originally he "possessed" vessels he deemed worthy of wielding him, but this also meant he dominated them until they died.
During Strive's Story, he reveals to A.B.A. that he doesn't think this is the correct way to do things and is willing to accept her as his partner even if they don't always connect with one another.
So during the command grab, he might try to "possess" a body, but A.B.A. pulls him out at the last minute, leaving him somewhat disformed and ambiguous (like the creatures that exist beyond the Gate he can summon).
He can still feel pain, especially if she bites him and scratches his face, but in that state he is extremely euphoric and can't really think straight... A.B.A. is also mutually affected by the possession as well, though it merely compounds her jealousy to twice the amount of what it was before the Union took place.
It also appears that being in that state greatly elevates A.B.A.'s physical strength, as previously she could barely lift Flament Nagel, but when under his influence, she can crack the very ground she stands on with sheer force of weight by how she wields him! This is evident in her victory animation in Strive.
I'll just stop here to say that we don't know Flament Nagel's full history during the Crusades, but he does remind me of another "weapon" that has a similar effect and namesake.
One of the Sacred Treasures: Jinki Ekitoku (Increasing Fervor), codenamed "Dominator" by the Conclave!
By that name alone, "Increasing Fervor" or even "Dominator" the effect appears very similar to the powers that Flament Nagel can wield...
I can't put my finger on it just yet, but I dare say there might be a connection with him and the Sacred Treasures (whether or not Sol Badguy created him is another mystery though).