Depression Moon has it, AP!

From the beginning, straight to the end, - monsters in all parts give AP
I'll be useful, you'll depend
On me for your survival, - you need AP to learn better healing skills
to help you beat your rival. - and better damage skills
But unless you verse a single, - A "single" refers to a boss, which you only verse a single time
I don't change for your tingle - most mobs give just 1 AP, while bosses give more typically
To learn all you can learn,
But trust me, you're not who I spurn. - It rhymed.

You're up, DM!