I think the easiest way to figure out how to implement abilities would be running a search (Ctrl+Shift+F) for itemvalidflag_AUTO_, this should bring up all the places where items are automatically triggered (e.g. the Spiked Shell being triggered on damage to deal a counter attack... you'd want a similar check for abilities being triggered on damage at basically the same place, for things like Flash Fire, Sturdy and so on; other abilities might trigger on each turn, or when attacking, etc)
You'll also need to actually give abilities to monsters (so init_monster needs to take a new argument for ability, the global.monster_data array be extended so it can fit this new data in, and the giant setup in init_monsters should provide it of course)
To show the ability, mev_pause_monsters_status is the script that builds the status screen. You'd read out the abl_NAME field here and create a ggui_element_text to display it somewhere.