Nope, it debuts in the f arc. It's basically a secondary meter you can activate at times for refilled CC and status healing, character-specific abilities, and a new attack. You'll get it from titles, which you could then carry over into your completed game or a new game +.
If you've ever played other Tales games, you can basically think of it like a unique, personalized overlimit.
While Eleth Burst/Break is effectively a whole-party overlimit, Accel Mode is a character specific version with unique effects all around. For example, Sophie gets absurd HP/CC regen as well as the ability to forward dash through opponents to combo from behind. Or Pascal's, easily my favorite, creates a barrier in which your team can't be staggered and takes laughable damage, while enemies cannot block at all.
You get Asbel's through the story for free. The rest you get through plot-relevant titles and need to be learned like any other skill.
PSN: Ranylyn. Currently: pissed at Gamestop Canada for Blazblue Continuum Shift Extend release date slaughtering.